Merge branch 'md_bold_italics' into dev

This fixes Markdown syntax for bold, italic, and bold_italic. With these
changes **bold** and __bold__, *italic* and _italic_, and
***bold_italic*** and ___italic_bold___ are shown correctly.
This closes #23, closes #189, closes #318, and closes #576.

doc/vimwiki.txt

@@ -877,6 +877,12 @@ is decorated: >
   super^script^
   sub,,script,,
 
+For Markdown syntax these variations are used: >
+
+  **bold text** or __bold text__
+  *italic text* or _italic text_
+  ***bold_italic text*** or ___italic_bold text___
+
 Furthermore, there are a number of words which are highlighted extra flashy:
 TODO, DONE, STARTED, FIXME, FIXED, XXX.