Add support for external rules

*   Add example rule, and rule object file, to
    `test/external`;
*   Add fixtures for external tests;
*   Move `mdast.js`, `mdast.min.js` to `mdast-lint.js`,
    `mdast-lint.min.js`;
*   Add docs for external rules.

Closes GH-14.

doc/rules.md

@@ -8,6 +8,7 @@ fix their warnings.
 
 *   [Rules](#rules)
 
+    *   [externals](#externals)
     *   [reset](#reset)
     *   [blockquote-indentation](#blockquote-indentation)
     *   [code-block-style](#code-block-style)
@@ -69,10 +70,27 @@ Remember that rules can always be turned off by
 passing false. In addition, when reset is given, values can
 be null or undefined in order to be ignored.
 
+### externals
+
+````md
+    <!-- Load more rules -->
+    ```json
+    {
+      "externals": ["foo", "bar", "baz"]
+    }
+    ```
+````
+
+Externals contains a list of extra rules to load.
+These are, or refer to, an object mapping `ruleId`s to rules.
+
+Note that in node.js, a string can be given (a module
+name or a file), but in the browser an object must be passed in.
+
 ### reset
 
 ````md
-  <!-- Explicitly activate rules: -->
+    <!-- Explicitly activate rules: -->
     ```json
     {
       "reset": true,

lib/index.js

@@ -14,10 +14,20 @@
 
 var range = require('mdast-range');
 var zone = require('mdast-zone');
-var rules = require('./rules');
+var internals = require('./rules');
 var sort = require('./sort');
 var filter = require('./filter');
 
+/*
+ * Needed for plug-in resolving.
+ */
+
+var path = require('path');
+var fs = require('fs');
+var exists = fs && fs.existsSync;
+var resolve = path && path.resolve;
+var cwd = process && process.cwd();
+
 /**
  * Factory to create a plugin from a rule.
  *
@@ -85,6 +95,78 @@ function attachFactory(id, rule, options) {
     return attach;
 }
 
+/**
+ * Require an external.  Checks, in this order:
+ *
+ * -  `$cwd/$pathlike`;
+ * -  `$cwd/$pathlike.js`;
+ * -  `$cwd/node_modules/$pathlike`;
+ * -  `$pathlike`.
+ *
+ * Where `$cwd` is the current working directory.
+ *
+ * @example
+ *   var plugin = findPlugin('foo');
+ *
+ * @throws {Error} - Fails when `pathlike` cannot be
+ *   resolved.
+ * @param {string} pathlike - Reference to external.
+ * @return {Object} - Result of `require`ing external.
+ */
+function loadExternal(pathlike) {
+    var local = resolve(cwd, pathlike);
+    var current = resolve(cwd, 'node_modules', pathlike);
+    var plugin;
+
+    if (exists(local) || exists(local + '.js')) {
+        plugin = local;
+    /* istanbul ignore else - for globals */
+    } else if (exists(current)) {
+        plugin = current;
+    } else {
+        plugin = pathlike;
+    }
+
+    return require(plugin);
+}
+
+/**
+ * Load all externals.  Merges them into a single rule
+ * object.
+ *
+ * In node, accepts externals as strings, otherwise,
+ * externals should be a list of objects.
+ *
+ * @param {Array.<string|Object>} externals
+ * @return {Array.<Object>}
+ * @throws {Error} - When an external cannot be resolved.
+ */
+function loadExternals(externals) {
+    var index = -1;
+    var rules = {};
+    var external;
+    var ruleId;
+    var mapping = externals ? externals.concat() : [];
+    var length;
+
+    mapping.push(internals);
+    length = mapping.length;
+
+    while (++index < length) {
+        external = mapping[index];
+
+        if (typeof external === 'string') {
+            external = loadExternal(external);
+        }
+
+        for (ruleId in external) {
+            rules[ruleId] = external[ruleId];
+        }
+    }
+
+    return rules;
+}
+
 /**
  * Lint attacher.
  *
@@ -105,6 +187,7 @@ function attachFactory(id, rule, options) {
 function lint(mdast, options) {
     var settings = options || {};
     var reset = settings.reset;
+    var rules = loadExternals(settings.external);
     var id;
     var setting;
 

mdast-lint.js

@@ -1,4 +1,4 @@
-(function(f){if(typeof exports==="object"&&typeof module!=="undefined"){module.exports=f()}else if(typeof define==="function"&&define.amd){define([],f)}else{var g;if(typeof window!=="undefined"){g=window}else if(typeof global!=="undefined"){g=global}else if(typeof self!=="undefined"){g=self}else{g=this}g.mdast = f()}})(function(){var define,module,exports;return (function e(t,n,r){function s(o,u){if(!n[o]){if(!t[o]){var a=typeof require=="function"&&require;if(!u&&a)return a(o,!0);if(i)return i(o,!0);var f=new Error("Cannot find module '"+o+"'");throw f.code="MODULE_NOT_FOUND",f}var l=n[o]={exports:{}};t[o][0].call(l.exports,function(e){var n=t[o][1][e];return s(n?n:e)},l,l.exports,e,t,n,r)}return n[o].exports}var i=typeof require=="function"&&require;for(var o=0;o<r.length;o++)s(r[o]);return s})({1:[function(require,module,exports){
+(function(f){if(typeof exports==="object"&&typeof module!=="undefined"){module.exports=f()}else if(typeof define==="function"&&define.amd){define([],f)}else{var g;if(typeof window!=="undefined"){g=window}else if(typeof global!=="undefined"){g=global}else if(typeof self!=="undefined"){g=self}else{g=this}g.mdastLint = f()}})(function(){var define,module,exports;return (function e(t,n,r){function s(o,u){if(!n[o]){if(!t[o]){var a=typeof require=="function"&&require;if(!u&&a)return a(o,!0);if(i)return i(o,!0);var f=new Error("Cannot find module '"+o+"'");throw f.code="MODULE_NOT_FOUND",f}var l=n[o]={exports:{}};t[o][0].call(l.exports,function(e){var n=t[o][1][e];return s(n?n:e)},l,l.exports,e,t,n,r)}return n[o].exports}var i=typeof require=="function"&&require;for(var o=0;o<r.length;o++)s(r[o]);return s})({1:[function(require,module,exports){
 'use strict';
 
 module.exports = require('./lib');
@@ -94,10 +94,20 @@ module.exports = attacher;
 
 var range = require('mdast-range');
 var zone = require('mdast-zone');
-var rules = require('./rules');
+var internals = require('./rules');
 var sort = require('./sort');
 var filter = require('./filter');
 
+/*
+ * Needed for plug-in resolving.
+ */
+
+var path = require('path');
+var fs = require('fs');
+var exists = fs && fs.existsSync;
+var resolve = path && path.resolve;
+var cwd = process && process.cwd();
+
 /**
  * Factory to create a plugin from a rule.
  *
@@ -165,6 +175,78 @@ function attachFactory(id, rule, options) {
     return attach;
 }
 
+/**
+ * Require an external.  Checks, in this order:
+ *
+ * -  `$cwd/$pathlike`;
+ * -  `$cwd/$pathlike.js`;
+ * -  `$cwd/node_modules/$pathlike`;
+ * -  `$pathlike`.
+ *
+ * Where `$cwd` is the current working directory.
+ *
+ * @example
+ *   var plugin = findPlugin('foo');
+ *
+ * @throws {Error} - Fails when `pathlike` cannot be
+ *   resolved.
+ * @param {string} pathlike - Reference to external.
+ * @return {Object} - Result of `require`ing external.
+ */
+function loadExternal(pathlike) {
+    var local = resolve(cwd, pathlike);
+    var current = resolve(cwd, 'node_modules', pathlike);
+    var plugin;
+
+    if (exists(local) || exists(local + '.js')) {
+        plugin = local;
+    /* istanbul ignore else - for globals */
+    } else if (exists(current)) {
+        plugin = current;
+    } else {
+        plugin = pathlike;
+    }
+
+    return require(plugin);
+}
+
+/**
+ * Load all externals.  Merges them into a single rule
+ * object.
+ *
+ * In node, accepts externals as strings, otherwise,
+ * externals should be a list of objects.
+ *
+ * @param {Array.<string|Object>} externals
+ * @return {Array.<Object>}
+ * @throws {Error} - When an external cannot be resolved.
+ */
+function loadExternals(externals) {
+    var index = -1;
+    var rules = {};
+    var external;
+    var ruleId;
+    var mapping = externals ? externals.concat() : [];
+    var length;
+
+    mapping.push(internals);
+    length = mapping.length;
+
+    while (++index < length) {
+        external = mapping[index];
+
+        if (typeof external === 'string') {
+            external = loadExternal(external);
+        }
+
+        for (ruleId in external) {
+            rules[ruleId] = external[ruleId];
+        }
+    }
+
+    return rules;
+}
+
 /**
  * Lint attacher.
  *
@@ -185,6 +267,7 @@ function attachFactory(id, rule, options) {
 function lint(mdast, options) {
     var settings = options || {};
     var reset = settings.reset;
+    var rules = loadExternals(settings.external);
     var id;
     var setting;
 
@@ -322,7 +405,7 @@ function lint(mdast, options) {
 
 module.exports = lint;
 
-},{"./filter":2,"./rules":18,"./sort":58,"mdast-range":64,"mdast-zone":65}],4:[function(require,module,exports){
+},{"./filter":2,"./rules":18,"./sort":58,"fs":undefined,"mdast-range":64,"mdast-zone":65,"path":undefined}],4:[function(require,module,exports){
 /**
  * @author Titus Wormer
  * @copyright 2015 Titus Wormer. All rights reserved.

package.json

@@ -55,8 +55,8 @@
     "make": "npm run lint && npm run test-coverage",
     "build-rules": "node script/build-rule-documentation.js",
     "build-md": "mdast . LICENSE --output",
-    "build-bundle": "browserify index.js -s mdast > mdast.js",
-    "postbuild-bundle": "esmangle mdast.js > mdast.min.js",
+    "build-bundle": "browserify index.js --bare -s mdastLint > mdast-lint.js",
+    "postbuild-bundle": "esmangle mdast-lint.js > mdast-lint.min.js",
     "build": "npm run build-rules && npm run build-md && npm run build-bundle",
     "prepublish": "npm run build"
   }

script/additional.json

@@ -2,5 +2,9 @@
   "reset": {
     "description": "By default, all rules are turned on unless explicitly set to `false`.\nWhen `reset: true`, the opposite is true: all rules are turned off,\nunless when given a non-nully and non-false value.\n\nOptions: `boolean`, default: `false`.",
     "example": "    <!-- Explicitly activate rules: -->\n    ```json\n    {\n      \"reset\": true,\n      \"final-newline\": true\n    }\n    ```\n"
+  },
+  "externals": {
+    "description": "Externals contains a list of extra rules to load.\nThese are, or refer to, an object mapping `ruleId`s to rules.\n\nNote that in node.js, a string can be given (a module\nname or a file), but in the browser an object must be passed in.",
+    "example": "    <!-- Load more rules -->\n    ```json\n    {\n      \"externals\": [\"foo\", \"bar\", \"baz\"]\n    }\n    ```\n"
   }
 }

test/external/index.js

@@ -0,0 +1,16 @@
+/**
+ * @author Titus Wormer
+ * @copyright 2015 Titus Wormer. All rights reserved.
+ * @module External
+ * @fileoverview Map of example external rules.
+ */
+
+'use strict';
+
+/*
+ * Expose.
+ */
+
+module.exports = {
+    'no-lorem': require('./no-lorem')
+};

test/external/no-lorem.js

@@ -0,0 +1,40 @@
+/**
+ * @author Titus Wormer
+ * @copyright 2015 Titus Wormer. All rights reserved.
+ * @module no-tabs
+ * @fileoverview
+ *   Warn when `lorem` is used in a document.
+ * @example
+ *   <!-- Invalid: -->
+ *   lorem
+ *
+ *   <!-- Valid: -->
+ *   ipsum
+ */
+
+'use strict';
+
+/**
+ * Warn when `lorem` is used in a document.
+ *
+ * @param {Node} ast - Root node.
+ * @param {File} file - Virtual file.
+ * @param {*} preferred - Ignored.
+ * @param {Function} done - Callback.
+ */
+function noLorem(ast, file, preferred, done) {
+    var content = file.toString();
+    var expression = /\blorem\b/gi;
+
+    while (expression.exec(content)) {
+        file.warn('Do not use lorem', file.offsetToPosition(expression.lastIndex));
+    }
+
+    done();
+}
+
+/*
+ * Expose.
+ */
+
+module.exports = noLorem;

test/fixtures/lorem-invalid.md

@@ -0,0 +1 @@
+Lorem.

test/fixtures/lorem-valid.md

@@ -0,0 +1 @@
+Ipsum.

test/index.js

@@ -206,6 +206,60 @@ describe('mdast-lint', function () {
     });
 });
 
+/*
+ * Validate external rules.
+ */
+
+describe('External', function () {
+    describe('Load external rules with require', function () {
+        assertFile('lorem-invalid.md', [
+            'lorem-invalid.md:1:1: Do not use lorem'
+        ], null, {
+            'external': ['test/external']
+        });
+    });
+
+    describe('Load external rules with require and without `.js` extension', function () {
+        assertFile('lorem-invalid.md', [
+            'lorem-invalid.md:1:1: Do not use lorem'
+        ], null, {
+            'external': ['test/external/index']
+        });
+    });
+
+    describe('Load external rules with require and with `.js` extension', function () {
+        assertFile('lorem-invalid.md', [
+            'lorem-invalid.md:1:1: Do not use lorem'
+        ], null, {
+            'external': ['test/external/index.js']
+        });
+    });
+
+    describe('Load local external rules', function () {
+        it('should fail on invalid external rules', function () {
+            assert.throws(function () {
+                process('lorem-valid.md', {
+                    'external': ['mdast']
+                });
+            });
+        });
+    });
+
+    describe('Load external rules by passing in', function () {
+        var external = require('./external');
+
+        assertFile('lorem-invalid.md', [
+            'lorem-invalid.md:1:1: Do not use lorem'
+        ], null, {
+            'external': [external]
+        });
+    });
+});
+
+/*
+ * Validate inline en- and disabling.
+ */
+
 describe('Comments', function () {
     describe('Disable and re-enable rules based on markers', function () {
         assertFile('comments-disable.md', [