Add Trac Support

Add support for Edgewall Trac. Tested in versions 0.12 - 1.1.2dev of
Trac. Custom styles added for Trac v0.x, Trac v1.x, and WordPress Trac.
For now, supported domains are trac.edgewall.org, trac-hacks.org,
trac.wordpress.org, and bugs.jquery.org. We may want to add to this list
in the future or allow users to add their own custom domains in
extension options.
Now I remember why I didn't use those generic patterns in the first place - Google doesn't allow them. :frowning: Specifically, `subdomain.*` is not allowed (even though `*.domain` and `http://*/*` are). Details are here: https://developer.chrome.com/extensions/match_patterns.

So, the only hope of allowing more sites is really just allowing users to add custom domains. I am on vacation until September, but I'll take a crack at a settings page for that when I get back!