Allow ReferenceURL and ReferenceLabel inside headers

Closes #260

Author: ErickChacon

syntax/pandoc.vim

@@ -367,7 +367,7 @@ call s:WithConceal("strikeout", 'syn match pandocStrikeoutMark /\~\~/ contained
 " }}}2
 
 " Headers: {{{2
-syn match pandocAtxHeader /\(\%^\|<.\+>.*\n\|^\s*\n\)\@<=#\{1,6}.*\n/ contains=pandocEmphasis,pandocStrong,pandocNoFormatted,pandocLaTeXInlineMath,@Spell,pandocAmpersandEscape display
+syn match pandocAtxHeader /\(\%^\|<.\+>.*\n\|^\s*\n\)\@<=#\{1,6}.*\n/ contains=pandocEmphasis,pandocStrong,pandocNoFormatted,pandocLaTeXInlineMath,@Spell,pandocAmpersandEscape,pandocReferenceLabel,pandocReferenceURL display
 syn match pandocAtxHeaderMark /\(^#\{1,6}\|\\\@<!#\+\(\s*.*$\)\@=\)/ contained containedin=pandocAtxHeader
 call s:WithConceal("atx", 'syn match pandocAtxStart /#/ contained containedin=pandocAtxHeaderMark', 'conceal cchar='.s:cchars["atx"])
 syn match pandocSetexHeader /^.\+\n[=]\+$/ contains=pandocEmphasis,pandocStrong,pandocNoFormatted,pandocLaTeXInlineMath,@Spell,pandocAmpersandEscape