Add option to allow tight lists in no-missing-blank-lines

This new feature adds support for allowing missing blank lines between
block-level nodes in lists, through passing `{exceptTightLists: true}`
to `no-missing-blank-lines` (default: false).

Closes GH-85.

doc/rules.md

@@ -1790,7 +1790,11 @@ mailto:qux@quux.com
 
 ## `no-missing-blank-lines`
 
-Warn for missing blank lines before a block node.
+Warn when missing blank lines before a block node.
+
+This rule can be configured to allow tight list items
+without blank lines between their contents through
+`exceptTightLists: true` (default: false).
 
 When this rule is turned on, the following file
 `valid.md` is ok:
@@ -1799,6 +1803,12 @@ When this rule is turned on, the following file
 # Foo
 
 ## Bar
+
+- Paragraph
+
+  + List.
+
+Paragraph.
 ```
 
 When this rule is turned on, the following file
@@ -1807,6 +1817,29 @@ When this rule is turned on, the following file
 ```markdown
 # Foo
 ## Bar
+
+- Paragraph
+  + List.
+
+Paragraph.
+```
+
+```text
+2:1-2:7: Missing blank line before block node
+5:3-5:10: Missing blank line before block node
+```
+
+When this rule is `{ exceptTightLists: true }`, the following file
+`tight.md` is **not** ok:
+
+```markdown
+# Foo
+## Bar
+
+- Paragraph
+  + List.
+
+Paragraph.
 ```
 
 ```text

lib/rules/no-missing-blank-lines.js

@@ -4,7 +4,11 @@
  * @license MIT
  * @module no-missing-blank-lines
  * @fileoverview
- *   Warn for missing blank lines before a block node.
+ *   Warn when missing blank lines before a block node.
+ *
+ *   This rule can be configured to allow tight list items
+ *   without blank lines between their contents through
+ *   `exceptTightLists: true` (default: false).
  *
  * @example {"name": "valid.md"}
  *
@@ -12,14 +16,40 @@
  *
  *   ## Bar
  *
+ *   - Paragraph
+ *
+ *     + List.
+ *
+ *   Paragraph.
+ *
  * @example {"name": "invalid.md", "label": "input"}
  *
  *   # Foo
  *   ## Bar
  *
+ *   - Paragraph
+ *     + List.
+ *
+ *   Paragraph.
+ *
  * @example {"name": "invalid.md", "label": "output"}
  *
  *   2:1-2:7: Missing blank line before block node
+ *   5:3-5:10: Missing blank line before block node
+ *
+ * @example {"name": "tight.md", "setting": {"exceptTightLists": true}, "label": "input"}
+ *
+ *   # Foo
+ *   ## Bar
+ *
+ *   - Paragraph
+ *     + List.
+ *
+ *   Paragraph.
+ *
+ * @example {"name": "tight.md", "setting": {"exceptTightLists": true}, "label": "output"}
+ *
+ *   2:1-2:7: Missing blank line before block node
  */
 
 'use strict';
@@ -38,7 +68,9 @@ module.exports = noMissingBlankLines;
  * @param {Node} ast - Root node.
  * @param {File} file - Virtual file.
  */
-function noMissingBlankLines(ast, file) {
+function noMissingBlankLines(ast, file, options) {
+  var allow = (options || {}).exceptTightLists;
+
   visit(ast, function (node, index, parent) {
     var next = parent && parent.children[index + 1];
 
@@ -46,10 +78,14 @@ function noMissingBlankLines(ast, file) {
       return;
     }
 
+    if (allow && parent && parent.type === 'listItem') {
+      return;
+    }
+
     if (
       next &&
-      isApplicable(node) &&
-      isApplicable(next) &&
+      applicable(node) &&
+      applicable(next) &&
       position.start(next).line === position.end(node).line + 1
     ) {
       file.warn('Missing blank line before block node', next);
@@ -63,7 +99,7 @@ function noMissingBlankLines(ast, file) {
  * @param {Node} node - Node to test.
  * @return {boolean} - Whether or not `node` is applicable.
  */
-function isApplicable(node) {
+function applicable(node) {
   return [
     'paragraph',
     'blockquote',