Support custom ENVs

Hey,

so I started trying to implement the proposal from https://gist.github.com/rwjblue/51f1df4b21a23466ae47
To make sure I understood it right and to show up my problems I'm submitting this in a really early hacky stage.

Instead of adding methods to the EmberApp object I created a config model that is passed to the respective configuration module.

See the corresponding config files here: https://github.com/dschmidt/ember-cli-sass-demo/tree/customenvs/config

Going to add my questions as comments to the code below


Best regards,
Dominik

^^ `get('hinting.js.app')` would be great, but we can add it in a follow up PR