add decorates property to controllers

This is a follow up on this : 
https://github.com/emberjs/ember.js/commit/ae9d164f0f54f5c0cf53355003c1ba9f0e8602de#commitcomment-2547456

I do not particularly like the name `decorates`, so I am up for better suggestions.

If there is enough interest I will add docs/tests

cc @tomdale