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.
Good suggestion. That would cover 4 of the 6 examples I gave with the most significant exclusion being http://trac-hacks.org/. Since that is one of the most popular Trac instances out there, it may warrant its own line. Also, consider these examples:

- http://bugs.icu-project.org/trac
- https://projects.appropriatesoftware.org/kforge/trac
- https://svn.boost.org/trac/boost

If we are going the route of generic URL patterns, we may want to at least make it something like:

`"*://trac.*/*",`
`"*://*.*/trac",`
`"*://*.*/trac/*",`
`"*://trac-hacks.org/*",`

Still, I think it would be great to allow users to add custom domains in a settings menu. @Toggl folks: If I were to submit a pull request for this feature, is it something you would consider adding?