Feature need: handle new Heroku pipeline review app convention

[geoffharcourt]

Heroku's new review apps system no longer names review applications with the same number as the Github PR in a predictable manner. From my conversations with their support team there's no way we can easily run a PR app without knowing the app suffix or for us to also connect to the Github API.

[geoffharcourt]

We're currently getting around some of this with a custom app we built that connects to Heroku's API and Github's and links the things together by branch name, but for CLI use I think the best approach would be this:

• start from the branch connected to the pull request
• type pr_app run rake middleware or whatever you want to do
• parity hits the Heroku and maybe GH APIs to route to the right spot

[geoffharcourt]

Heroku's added the ability to go back to your own PR app names based on PR numbers now, so this is less necessary.