Modify horizontal rule (thematic-breaks) syntax for markdown.

This change makes the horizontal rule syntax more closely match the
commonmark spec https://spec.commonmark.org/0.29/#thematic-breaks. The
number of characters required was changed to 3 and '___' and '***' where
added as valid sequences. The space rules were not added becuase it adds
extra complexity for little benefit. Issue #664.

syntax/vimwiki_markdown.vim

@@ -67,7 +67,7 @@ let s:markdown_syntax.symH = 0
 
 
 " <hr>, horizontal rule
-let s:markdown_syntax.rxHR = '^-----*$'
+let s:markdown_syntax.rxHR = '\(^---*$\|^___*$\|^\*\*\**$\)'
 
 " Tables. Each line starts and ends with '|'; each cell is separated by '|'
 let s:markdown_syntax.rxTableSep = '|'