RFC: ES6 container

[RFC document](https://github.com/MajorBreakfast/rfcs/blob/master/active/0000-es6-container.md)
the way we use them in ember-cli is fine. The idea is for us to allow users to add formal "slabs" of there app as siblings, ala:

```
/app/*
/admin/*
/config/*
```

this will not only facilitate clear and concise lazy trees, but will easily facility multi-repo or multi-team development experiences. It becomes immediately obvious of admin and config are truly independent, and tooling can be written to statically valid based on obvious intent.