[Review Requested] Add scaffold_controller generator

This is a PR request to integrate Roar with Rails' `scaffold_controller` functionality. This is a feature in both [jbuilder](https://github.com/rails/jbuilder) and [active_model_serializers](https://github.com/rails-api/active_model_serializers) that I liked, so I decided to do a Roar-inspired version of it.

When a user executes `rails generate scaffold_controller Model`, it will now build out the stubbing for Roar including controller methods and Representers.

I am asking for a some feedback on this because I do not know 100% if my default settings would be considered optimal, and I would love feedback to know what would be considered most 'sane' so users can get the best Roar-Rails experience with minimal effort. Please please please let me know if you think things can be done better and I will fix it! I have enjoyed this Gem greatly so glad I can help.

Should we go the `Controller::represents` way here and set this on the class level instead of using `:represent_with` in every action? This option was meant to be a work-around if you need to customise an edge case.

Do you use `::represents` at all?