Allow addons to customize es6 module prefix

Addons can now customize the module prefix they are compiled under:

```js
// index.js

module.exports = {
  name: 'some-addon',
  modulePrefix: 'other-addon'
}
```

Normally addons would use `name` to compile under. If `modulePrefix` is
declared that is used instead.

Use Case!!

During the ember-validations rewrite I've realized that I'd like to have
another library: ember-validations-addons or something to that effect.
It will contain additional validators that don't need to be part of the
original library. (like the URI validator) But I still need to lookup
validators in the `ember-validations` namespace rather than attempting
to manage additional namespaces. This would allow others to also write
their own addons in the same namespace.
Merge pull request #2189 from bcardarella/bc-custom-addon-module-prefixes

Allow addons to customize es6 module prefix