Implement alternative views.

This patch adds support for alternative patch views for all the different
patch displays (commitdiff, filediff, headdiff and headfilediff), supporting
currently a plain output and, for commitdiff, a raw one.

A darcs output is included but commented out, since I'm not familiar with
darcs' native output enough to make a syntax highlighter for it.

Besides, it also includes an alternative view for headblob (I was too lazy to
split up the patch =). Thanks to Zachary P. Landau who sent a patch to do this
(although I did it in a slightly different way).