Added google drive integration

Not sure if you're interested in extending this beyond bug trackers, but this simple addition adds support for Google Docs.  I wrote it for the actual "docs" application and it ended up working for sheets, slides, forms, and drawings as well without modification!

Limitations / Notes
* No project support (not sure what that would correlate to in docs anyway, folder maybe?)
* The placement is admittedly a little squished, but I didn't want to mess with Google's stuff too much
* Having to add google.com to background.js means the toggl button will show up on every google.com property even though this only works for google docs.  But hey, we're all toggl enthusiasts here, right?