Allow pandocDelimitedCodeBlockStart at top of file (#345)

The regex looking for the start of a codeblock was looking for two
newlines before the triple backticks.

At the start of a file, that doesn't work, but it's still fine for
codeblocks to be there.

Author: jez

syntax/pandoc.vim

@@ -436,7 +436,7 @@ syn match pandocTableHeaderWord /\<.\{-}\>/ contained containedin=pandocGridTabl
 " this is here because we can override strikeouts and subscripts
 syn region pandocDelimitedCodeBlock start=/^\(>\s\)\?\z(\([ ]\{4,}\|\t\)\=\~\{3,}\~*\)/ end=/^\z1\~*/ skipnl contains=pandocDelimitedCodeBlockStart,pandocDelimitedCodeBlockEnd keepend
 syn region pandocDelimitedCodeBlock start=/^\(>\s\)\?\z(\([ ]\{4,}\|\t\)\=`\{3,}`*\)/ end=/^\z1`*/ skipnl contains=pandocDelimitedCodeBlockStart,pandocDelimitedCodeBlockEnd keepend
-call s:WithConceal('codeblock_start', 'syn match pandocDelimitedCodeBlockStart /\(\_^\n\_^\(>\s\)\?\([ ]\{4,}\|\t\)\=\)\@<=\(\~\{3,}\~*\|`\{3,}`*\)/ contained containedin=pandocDelimitedCodeBlock nextgroup=pandocDelimitedCodeBlockLanguage', 'conceal cchar='.s:cchars['codelang'])
+call s:WithConceal('codeblock_start', 'syn match pandocDelimitedCodeBlockStart /\(\(\_^\n\_^\|\%^\)\(>\s\)\?\([ ]\{4,}\|\t\)\=\)\@<=\(\~\{3,}\~*\|`\{3,}`*\)/ contained containedin=pandocDelimitedCodeBlock nextgroup=pandocDelimitedCodeBlockLanguage', 'conceal cchar='.s:cchars['codelang'])
 syn match pandocDelimitedCodeBlockLanguage /\(\s\?\)\@<=.\+\(\_$\)\@=/ contained
 call s:WithConceal('codeblock_delim', 'syn match pandocDelimitedCodeBlockEnd /\(`\{3,}`*\|\~\{3,}\~*\)\(\_$\n\(>\s\)\?\_$\)\@=/ contained containedin=pandocDelimitedCodeBlock', 'conceal cchar='.s:cchars['codeend'])
 syn match pandocBlockQuoteinDelimitedCodeBlock '^>' contained containedin=pandocDelimitedCodeBlock