unified diff filter

when generating patch files in unified diff format, i find them much easier to read when they are color-coded. i have a text editor that provides this functionality and it's quite handy.

i'm proposing that we add a 'Unified diff' filter to the module, so that, for example, drupalbin.com can support color-coded patch posts.

my initial thought is simply to color added lines green, subtracted lines red, context lines black, and possibly the file/line number references at the beginning of a hunk blue.