Never conceal \ in \newcommand... (#197)

... even if pandoc#syntax#conceal#backslash is set.

Test with the following file:

    \*foo
    \renewcommand
    \foo

Fixes #167.

Author: anntzer

syntax/pandoc.vim

@@ -470,7 +470,7 @@ endif
 syn match pandocHRule /^\s*\([*\-_]\)\s*\%(\1\s*\)\{2,}$/ display
 " Backslashes: {{{3
 if g:pandoc#syntax#conceal#backslash == 1
-    syn match pandocBackslash /\\\@<!\\/ containedin=ALLBUT,pandocCodeblock,pandocCodeBlockInsideIndent,pandocNoFormatted,pandocNoFormattedInEmphasis,pandocNoFormattedInStrong,pandocDelimitedCodeBlock,pandocLineBlock,pandocYAMLHeader conceal
+    syn match pandocBackslash /\v\\@<!\\((re)?newcommand)@!/ containedin=ALLBUT,pandocCodeblock,pandocCodeBlockInsideIndent,pandocNoFormatted,pandocNoFormattedInEmphasis,pandocNoFormattedInStrong,pandocDelimitedCodeBlock,pandocLineBlock,pandocYAMLHeader conceal
 endif
 " }}}
 " }}}