[WIP] es6 namespaced addons

This will allow one to have modules in their addons that are namespaced
under their addon's name rather than the application's namespace.
Expects the following directory structures:

addon
└── mixins
    └── data-route.js
app
└── controllers
    └── people.js
Any modules in app/ will always be namespaced under whatever namespace
the parent application has. This allows the addon developer to create
common-sense defaults that get pulled into the application then can be
overridden by the consuming developer in the parent project's app/
directory.

Any modules in addon/ will always be namespaced under the addon's name.
This name is always forced to lower-cased & hyphenated. So:

"Ember CLI Some Addon" => "ember-cli-some-addon"
Then in whichever modules the consuming developer wants to use this
module they can do:

import DataRouteMixin from "ember-cli-some-addon/mixins/data-route";
This feature will traverse the addon/ directory and build a list of
modules from the available file names and merge this into
app.importWhitelist after the addon's modules have been compiled to ES6.

This feature will compile the addon's assets into vendor.js and
vendor.css. It does this by expecting the addon to compile its own
assets into a single file then all addon's files are concat'ed into an
addon.js then eventually concat'ed into vendor.js.

The EmberApp's registry is no passed into all preprocessors. This allows
the Addon to manage its own registry rather than rely on the private singleton.