Pluggable API options for Hosted Projects

More git hosting services are popping up all the time, some of which have an API (e.g. Unfuddle, Codebasehq.com), and also many people self-host on gitweb, cgit and potentially others. We can probably never keep up with every service explicitly, so it would be nice to have a way for the user to plug in some kind of hook to support whatever API they wanted, so that their projects will appear on the Hosted Projects view (of course, any hosting service at all can still be used regardless just by using the clone/remote URL).

Probably don't want to make this a native plugin, because that's trickier for people to write and also might have security issues. Ideally some kind of scripting hook might be best. Don't want to add a whole scripting engine to SourceTree just for this, maybe we can repurpose AppleScript, Python or even just call out to a shell script of the user's choice.