Add support for global externals when running globally

Closes GH-38.
Closes GH-39.

doc/rules.md

@@ -93,6 +93,9 @@ 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.
 
+When using a globally installed remark-lint, globally installed external
+rules are also loaded.
+
 ### reset
 
 ````md

lib/index.js

@@ -21,6 +21,7 @@ var range = require('remark-range');
 var zone = require('mdast-zone');
 var internals = require('./rules');
 var filter = require('./filter');
+var npmPrefix = require('npm-prefix')();
 
 /*
  * Needed for plug-in resolving.
@@ -30,12 +31,24 @@ var path = require('path');
 var fs = require('fs');
 var exists = fs && fs.existsSync;
 var resolve = path && path.resolve;
+var isWindows;
+var isGlobal;
+var globals;
 var cwd;
 
+var MODULES = 'node_modules';
+
 /* istanbul ignore else */
 if (typeof global !== 'undefined') {
     /* global global */
     cwd = global.process.cwd();
+
+    /* Detect whether we’re running as a globally installed package. */
+    isWindows = global.process.platform === 'win32';
+    isGlobal = global.process.argv[1].indexOf(npmPrefix) === 0;
+
+    /* istanbul ignore next */
+    globals = resolve(npmPrefix, isWindows ? '' : 'lib', MODULES);
 }
 
 /**
@@ -117,6 +130,14 @@ function attachFactory(id, rule, options) {
  *
  * Where `$cwd` is the current working directory.
  *
+ * When using a globally installed executable, the
+ * following are also included:
+ *
+ * -  `$globals/$pathlike`.
+ *
+ * Where `$globals` is the directory of globally installed
+ * npm packages.
+ *
  * @example
  *   var plugin = findPlugin('foo');
  *
@@ -128,6 +149,7 @@ function attachFactory(id, rule, options) {
 function loadExternal(pathlike) {
     var local = resolve(cwd, pathlike);
     var current = resolve(cwd, 'node_modules', pathlike);
+    var globalPath = resolve(globals, pathlike);
     var plugin;
 
     if (exists(local) || exists(local + '.js')) {
@@ -135,6 +157,8 @@ function loadExternal(pathlike) {
     /* istanbul ignore else - for globals */
     } else if (exists(current)) {
         plugin = current;
+    } else if (isGlobal && exists(globalPath)) {
+        plugin = globalPath;
     } else {
         plugin = pathlike;
     }

package.json

@@ -18,6 +18,7 @@
     "mdast-util-position": "^1.0.0",
     "mdast-util-to-string": "^1.0.0",
     "mdast-zone": "^2.0.0",
+    "npm-prefix": "^1.1.1",
     "plur": "^2.0.0",
     "unist-util-visit": "^1.0.0",
     "vfile-sort": "^1.0.0"

script/additional.json

@@ -4,7 +4,7 @@
     "example": "<!-- Explicitly activate rules: -->\n```json\n{\n  \"reset\": true,\n  \"final-newline\": true\n}\n```\n"
   },
   "external": {
-    "description": "External 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.",
+    "description": "External 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.\n\nWhen using a globally installed remark-lint, globally installed external\nrules are also loaded.",
     "example": "<!-- Load more rules -->\n```json\n{\n  \"external\": [\"foo\", \"bar\", \"baz\"]\n}\n```\n"
   }
 }