Support tilde code blocks

Also ensure the end marker has the same number of delimiter characters
as the start marker.

Closes #156

Author: tpope

syntax/markdown.vim

@@ -106,7 +106,8 @@ exe 'syn region markdownBoldItalic matchgroup=markdownBoldItalicDelimiter start=
 
 syn region markdownCode matchgroup=markdownCodeDelimiter start="`" end="`" keepend contains=markdownLineStart
 syn region markdownCode matchgroup=markdownCodeDelimiter start="`` \=" end=" \=``" keepend contains=markdownLineStart
-syn region markdownCode matchgroup=markdownCodeDelimiter start="^\s*````*.*$" end="^\s*````*\ze\s*$" keepend
+syn region markdownCode matchgroup=markdownCodeDelimiter start="^\s*\z(`\{3,\}\).*$" end="^\s*\z1\ze\s*$" keepend
+syn region markdownCode matchgroup=markdownCodeDelimiter start="^\s*\z(\~\{3,\}\).*$" end="^\s*\z1\ze\s*$" keepend
 
 syn match markdownFootnote "\[^[^\]]\+\]"
 syn match markdownFootnoteDefinition "^\[^[^\]]\+\]:"
@@ -117,7 +118,8 @@ if main_syntax ==# 'markdown'
     if has_key(s:done_include, matchstr(s:type,'[^.]*'))
       continue
     endif
-    exe 'syn region markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*````*\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*````*\ze\s*$" keepend contains=@markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\.','','g') . s:concealends
+    exe 'syn region markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(`\{3,\}\)*\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\.','','g') . s:concealends
+    exe 'syn region markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\..*','','').' matchgroup=markdownCodeDelimiter start="^\s*\z(\~\{3,\}\)*\s*\%({.\{-}\.\)\='.matchstr(s:type,'[^=]*').'}\=\S\@!.*$" end="^\s*\z1\ze\s*$" keepend contains=@markdownHighlight'.substitute(matchstr(s:type,'[^=]*$'),'\.','','g') . s:concealends
     let s:done_include[matchstr(s:type,'[^.]*')] = 1
   endfor
   unlet! s:type