Merge pull request #660 from KSR-Yasuda/bugfix/ContainCodeInHeading

Should contain `code` in headings

Author: shirosaki

syntax/markdown.vim

@@ -156,7 +156,7 @@ if get(g:, 'vim_markdown_strikethrough', 0)
     HtmlHiLink mkdStrike        htmlStrike
 endif
 
-syn cluster mkdHeadingContent contains=htmlItalic,htmlBold,htmlBoldItalic,mkdFootnotes,mkdLink,mkdInlineURL,mkdStrike
+syn cluster mkdHeadingContent contains=htmlItalic,htmlBold,htmlBoldItalic,mkdFootnotes,mkdLink,mkdInlineURL,mkdStrike,mkdCode
 syn cluster mkdNonListItem contains=@htmlTop,htmlItalic,htmlBold,htmlBoldItalic,mkdFootnotes,mkdInlineURL,mkdLink,mkdLinkDef,mkdLineBreak,mkdBlockquote,mkdCode,mkdRule,htmlH1,htmlH2,htmlH3,htmlH4,htmlH5,htmlH6,mkdMath,mkdStrike
 
 "highlighting for Markdown groups