Regex and styling fixes

* Hopefully fixed a bug with the macro regex that hindered performance
badly when an unclosed << occurred before a decent number of " marks.
* Also fixed the bold, underline and italic syntax highlighting.
* Removed the 'labeldefault' parameter from <<back>>/<<return>>, on the
basis that it was largely undocumented, unusable due to a bug, and (the
real reason) it doesn't work with state serialisation.