Add space_in_atx_header extension.

This is enabled by default in pandoc and GitHub markdown but not the
other flavors.

This requirse a space between the opening #'s and the header
text in ATX headers (as CommonMark does but many other implementations
do not).  This is desirable to avoid falsely capturing things ilke

    #hashtag

or

    #5

Closes #3512.

MANUAL.txt

@@ -1634,6 +1634,12 @@ wrapping). Consider, for example:
     I like several of their flavors of ice cream:
     #22, for example, and #5.
 
+#### Extension: `space_in_atx_header` ####
+
+Many Markdown implementations do not require a space between the
+opening `#`s of an ATX header and the header text, so that
+`#5 bolt` and `#hashtag` count as headers.  With this extension,
+pandoc does require the space.
 
 ### Header identifiers ###
 

src/Text/Pandoc/Extensions.hs

@@ -112,6 +112,7 @@ data Extension =
     | Ext_intraword_underscores  -- ^ Treat underscore inside word as literal
     | Ext_blank_before_blockquote -- ^ Require blank line before a blockquote
     | Ext_blank_before_header     -- ^ Require blank line before a header
+    | Ext_space_in_atx_header -- ^ Require space between # and header text
     | Ext_strikeout           -- ^ Strikeout using ~~this~~ syntax
     | Ext_superscript         -- ^ Superscript using ^this^ syntax
     | Ext_subscript           -- ^ Subscript using ~this~ syntax
@@ -168,6 +169,7 @@ pandocExtensions = extensionsFromList
   , Ext_intraword_underscores
   , Ext_blank_before_blockquote
   , Ext_blank_before_header
+  , Ext_space_in_atx_header
   , Ext_strikeout
   , Ext_superscript
   , Ext_subscript
@@ -223,6 +225,7 @@ githubMarkdownExtensions = extensionsFromList
   , Ext_ascii_identifiers
   , Ext_backtick_code_blocks
   , Ext_autolink_bare_uris
+  , Ext_space_in_atx_header
   , Ext_intraword_underscores
   , Ext_strikeout
   , Ext_hard_line_breaks

src/Text/Pandoc/Readers/Markdown.hs

@@ -534,6 +534,7 @@ atxHeader = try $ do
   level <- atxChar >>= many1 . char >>= return . length
   notFollowedBy $ guardEnabled Ext_fancy_lists >>
                   (char '.' <|> char ')') -- this would be a list
+  guardDisabled Ext_space_in_atx_header <|> notFollowedBy nonspaceChar
   skipSpaces
   (text, raw) <- withRaw $
           trimInlinesF . mconcat <$> many (notFollowedBy atxClosing >> inline)

test/command/3512.md

@@ -0,0 +1,13 @@
+```
+% pandoc -f markdown-auto_identifiers
+#hi
+^D
+<p>#hi</p>
+```
+
+```
+% pandoc -f markdown-auto_identifiers-space_in_atx_header
+#hi
+^D
+<h1>hi</h1>
+```