`heroku fork` support

@ddollar, @mmcgrana - I wanted to push what progress I have so far just to make sure I'm on the right path integrating heroku-fork into the mainline CLI. Any warning flags so far?

Also, in looking ahead, what's the best way to see sample data packets from these calls (to mock out in my specs)? Just `curl` - or something a little slicker?

To be clear, I don't want to merge anything yet. Just keep this as an ongoing discussion of the integration effort.