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 thought of doing that (and you still can), but figured I would throw this in as well. For folks building addons that don't have a great knowledge of the internals of the CLI, it seemed a bit arbitrary that you get `app` passed in, but `parentAddon` is on `this`.

Basically, pure syntatic sugar. I can remove if you'd like.