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.
you removed the newline character at the end of the last line :)