#67 diff highlighting support

[Christian Thäter]

make a sub-highlighter which changes background color (is that possible?, alternatively just change foreground color and don't do normal highlighting) based on the first characters in a line, let the normal highlighting code ignore this first characters and special lines (+++ --- @@@).

Special points for git-conflict-diff highlighting,
git has a special diff mode for diffing merge conflicts,
this mode introduces an extra row that you have 2 markers at the beginning of the line, one for YOURS and one for THEIRS over a BASE:
@@@ line markers have triple at's
-- line got removed in YOURS and THEIRS
- YOU deleted, THEY retained the line
- YOU retained, THEY deleted
++ dito for added lines
+ ..
+ ..

I am currently working on a git web-frontend
(WIP: http://www.pipapo.org/webgit\) and plan to use codepress, this things would be make a very nice addition. Unfortunally I am busy with that and not very used to javascript, else I would add such on my own.

Thanks,
Christian