Collapsible font families

![foobar](https://cloud.githubusercontent.com/assets/83384/3449328/a486607e-0164-11e4-88ed-cb1cd0de9486.gif)

Background: https://github.com/mapbox/tm2/issues/498

@samanpwbb @nickidlugash @amyleew quick review? There's def some family grouping logic that I think we can tweak in the future but if this approach looks reasonable let's merge to address the DOM performance issue there is now.