Fix interference between simple tables and unordered lists

Closes #193

It's possible to have empty unordered list items like this:

-
-

Currently when there are at least two list items with at least one of
them being empty, pandoc-syntax interprets these as tables and hides
the empty item and all sub-items, empty or not, as well as some other
non-empty items at the same level.

This patch requires simple tables to be have dashed lines where each
group of dashes contains at least 2 hyphens, thus eliminating the clash
with empty unordered lists.

Author: bwhelm

syntax/pandoc.vim

@@ -349,13 +349,13 @@ syn match pandocLineBlockDelimiter /^|/ contained containedin=pandocLineBlock
 
 " Simple: {{{3
 
-syn region pandocSimpleTable start=/\%#=2\(^.*[[:graph:]].*\n\)\@<!\(^.*[[:graph:]].*\n\)\(-\+\s*\)\+\n\n\@!/ end=/\n\n/ containedin=ALLBUT,pandocDelimitedCodeBlock,pandocYAMLHeader keepend
+syn region pandocSimpleTable start=/\%#=2\(^.*[[:graph:]].*\n\)\@<!\(^.*[[:graph:]].*\n\)\(-\{2,}\s*\)\+\n\n\@!/ end=/\n\n/ containedin=ALLBUT,pandocDelimitedCodeBlock,pandocYAMLHeader keepend
 syn match pandocSimpleTableDelims /\-/ contained containedin=pandocSimpleTable
 syn match pandocSimpleTableHeader /\%#=2\(^.*[[:graph:]].*\n\)\@<!\(^.*[[:graph:]].*\n\)/ contained containedin=pandocSimpleTable
 
-syn region pandocTable start=/\%#=2^\(-\+\s*\)\+\n\n\@!/ end=/\%#=2^\(-\+\s*\)\+\n\n/ containedin=ALLBUT,pandocDelimitedCodeBlock,pandocYAMLHeader keepend
+syn region pandocTable start=/\%#=2^\(-\{2,}\s*\)\+\n\n\@!/ end=/\%#=2^\(-\{2,}\s*\)\+\n\n/ containedin=ALLBUT,pandocDelimitedCodeBlock,pandocYAMLHeader keepend
 syn match pandocTableDelims /\-/ contained containedin=pandocTable
-syn region pandocTableMultilineHeader start=/\%#=2\(^-\+\n\)\@<=./ end=/\%#=2\n-\@=/ contained containedin=pandocTable
+syn region pandocTableMultilineHeader start=/\%#=2\(^-\{2,}\n\)\@<=./ end=/\%#=2\n-\@=/ contained containedin=pandocTable
 
 " }}}3
 " Grid: {{{3