Enable properly nested addons

See #2354 for context.

This adds a `parentAddon` context for any nested addons included in a project. The `parentAddon` is supplied to each child addon on the addon instance as well as passed into the `included` hook.

The child addon is then free to determine if it should modify the `parentAddon` or the `app` from within the `included` hook.

This enables addons to have their own isolated dependencies which don't interfere with the root app's preprocessors. I.e. I can write my addon in Coffeescript, and the parent app doesn't need to depend on the Coffeescript addon directly.
I think that we need to automatically handle managing nested addons' registry. When an addon is included, it should call `included` on all of it's "children" and pass itself as the "app".

Basically, an addon should always be calling `.registry.add` on whatever the first argument is and we should manage that properly internally.