Upgrade to the v6 structure

[gulopine]

Heroku CLI v6 plugins use a very different code format for defining commands and command options. We should upgrade to this structure as well, to start taking advantage of newer features.

In particular, command flags can now be complicated pieces of functionality that can intelligently choose defaults, interactively prompt for options and provide custom objects to the rest of the function. The app flag is a good example of this, and can serve as a template for how we can reimplement our resource flag, and perhaps contribute it back to the core codebase once we have a good implementation.