Support resizing in the ps:scale command.

This isn't ready to merge yet; though it works fine in manual testing there are test failures going on that I don't understand.

https://travis-ci.org/heroku/heroku/jobs/14230481

Going to continue digging since I noticed Travis is showing a bunch of stack frames that mysteriously got elided from my local runs, but if there's anyone who's rspec-savvy who could give some pointers that would be great.