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.
Ok, that's a bit different than the approach I outlined previously in the gist.

Is there any use case for a nested addon to have access to the root app? That was the reason I went with this approach - I can't think of a good one off the top of my head, but I wasn't sure.

I'll work this in barring additional comments.