[Markdown] Fix ATX heading

This commit enforces at least one whitespace after a hashtag to match
a token as atx heading.

see: markedjs/marked#201

Markdown/Markdown.sublime-syntax

@@ -26,7 +26,7 @@ variables:
         )
     setext_escape: ^(?=\s{0,3}(?:---+|===+)\s*$)
     block_quote: (?:[ ]{,3}>(?:.|$))         # between 0 and 3 spaces, followed by a greater than sign, followed by any character or the end of the line
-    atx_heading: (?:[#]{1,6}\s*)             # between 1 and 6 hashes, followed by any amount of whitespace
+    atx_heading: (?:[#]{1,6}\s+)             # between 1 and 6 hashes, followed by any amount of whitespace
     indented_code_block: (?:[ ]{4}|\t)       # 4 spaces or a tab
     list_item: (?:[ ]{,3}(?=\d+\.|[*+-])\d*([*+.-])\s) # between 0 and 3 spaces, followed by either: at least one integer and a full stop, or (a star, plus or dash), followed by whitespace
     escape: '\\[-`*_#+.!(){}\[\]\\>|~<]'

Markdown/syntax_test_markdown.md

@@ -20,6 +20,10 @@ http://spec.commonmark.org/0.28/#example-44
 |^^^^^^^^^^^^^^^^^^^^^^ markup.heading
 |             ^ - punctuation.definition.heading.end.markdown
 
+#NotAHeading
+| <- - markup.heading
+|^^^^^^^^^^^^ - markup.heading
+
 Alternate Heading
 | <- markup.heading.1
 =================