Allow setting default Application IDs for sandbox and production in environment.js

[deanpapastrat]

Are these automatically set depending on if you're running ember locally vs making a production build?

I couldn't really find out where you would set your environment for Ember and whether it would make sense to have a .env file or config.json to set your Sandbox vs Production App-IDs. Otherwise, it would be good to clarify this in the Getting Started docs.

Originally posted by @mootrichard in #35