fixes in folding

Comments to all chunks:

1. A typo: extra parenthesis breaks all folding
2. There is no need for look-behind here because '^' is zero-width and
   len(matchstr(...)) will be same whereas matchstr(...) will trigger
   same way. (i already omitted such look-behind in MarkdownLevelSA())
3. There could be 6 '#' and one space: 7 bytes altogether
Merge pull request #26 from lyokha/master

fixes in folding