Fix syntax matching for bold, italic, underline, code and verbatim.

I noticed that matching bold, italic etc. syntax doesn't work correctly (same way as in Emacs) in all cases. For example, it didn't match \*bold\*   if first *  was at the beginning of line. It also did match (incorrectly) "foo_under lined_bar".

I fixed those cases. My solution has one problem, however. It doesn't work well with non ascii word chars because \w matches only [0-9A-Za-z_]. So bold doesn't work for example, for the word \*Übermensch\*.