Patch for code inside brackets breaking fulther code highlighting, fix 238 (#248)

Copy syntax rule for links from `$VIMRUNTIME/syntax/markdown.vim`.

Author: doronbehar

syntax/pandoc.vim

@@ -264,7 +264,7 @@ syn match pandocLinkTip /\s*".\{-}"/ contained containedin=pandocReferenceURL co
 call s:WithConceal("image", 'syn match pandocImageIcon /!\[\@=/ display', 'conceal cchar='. s:cchars["image"]) 
 " }}}
 " Definitions: {{{3
-syn region pandocReferenceDefinition start=/\[.\{-}\]:/ end=/\(\n\s*".*"$\|$\)/ keepend
+syn region pandocReferenceDefinition start="!\=\[\%(\_[^]]*]\%( \=[[(]\)\)\@=" end="\]\%( \=[[(]\)\@=" keepend
 syn match pandocReferenceDefinitionLabel /\[\zs.\{-}\ze\]:/ contained containedin=pandocReferenceDefinition display
 syn match pandocReferenceDefinitionAddress /:\s*\zs.*/ contained containedin=pandocReferenceDefinition
 syn match pandocReferenceDefinitionTip /\s*".\{-}"/ contained containedin=pandocReferenceDefinition,pandocReferenceDefinitionAddress contains=@Spell,pandocAmpersandEscape