Improve support for Coffeescript syntax.

What's new in this patch:

Highlight.js core: 

 - New modes options markBegin and markEnd : If true it will wrap the content of either the begin or end match into a span with the mode classname suffixed with either _begin or _end. 
 - New modes options  beginCaptures and endCaptures : It allow to highlight capture groups within the begin and end matches.
 - New modes options  returnBeginCapture and returnEndCapture : Works as the returnBegin and returnEnd options but will return to the corresponding capture group.

All theses options are optional and alter the highlight.js behavior only when set.

CoffeeScript: 
 - Regex subexpressions are now highlighted.
 - Content of an interpolation and function params are now highlighted.

Here the result on my syntax test file with a customized solarized dark theme: 
http://cedricnehemie.com/media/docs/highlight.js/sample-coffee.html