WIP: API docs metadata

This moves away from biscotto and to generation of API doc json data.

Metadata JSON is generated by [donna][donna], and then digested into something for easy html generation by [tello][tello]. The resulting JSON is checked in here. I am not sure what exactly we want to do (likely just commit the tello.json).

This cannot be merged until we are ready to move away from the biscotto docs. This work has been started in https://github.com/github/atom.io/issues/638

@thedaniel let me know how I can unblock/help you.