Fix number sign at end of heading (atx)

Closes GH-20.

Author: wooorm

lib/unsafe.js

@@ -36,6 +36,7 @@ module.exports = [
   {character: '"', inConstruct: 'titleQuote'},
   // A number sign could start an ATX heading if it starts a line.
   {atBreak: true, character: '#'},
+  {character: '#', inConstruct: 'headingAtx', after: '(?:[\r\n]|$)'},
   // Dollar sign and percentage are not used in markdown.
   // An ampersand could start a character reference.
   {character: '&', after: '[#A-Za-z]', inConstruct: 'phrasing'},

test.js

@@ -1078,6 +1078,24 @@ test('heading', function (t) {
     'should not escape a `>` at the start of phrasing in a heading'
   )
 
+  t.equal(
+    to({type: 'heading', children: [{type: 'text', value: 'a #'}]}),
+    '# a \\#\n',
+    'should escape a `#` at the end of a heading (1)'
+  )
+
+  t.equal(
+    to({type: 'heading', children: [{type: 'text', value: 'a ##'}]}),
+    '# a #\\#\n',
+    'should escape a `#` at the end of a heading (2)'
+  )
+
+  t.equal(
+    to({type: 'heading', children: [{type: 'text', value: 'a # b'}]}),
+    '# a # b\n',
+    'should not escape a `#` in a heading (2)'
+  )
+
   t.end()
 })