Add `item-controller` type.

This should be a completely backwards compatible change. The new format allows for the item-controller to fall within the new pod structure alongside the template/route/controller that it is used within.

For example:

```javascript
App.PostsRoute = Ember.Route.extend();
App.PostsController = Ember.Route.extend();
App.PostsItemController = Ember.Route.extend();
```

Could allow the following structure:

```
app/posts/route.js
app/posts/controller.js
app/posts/item-controller.js
app/posts/template.hbs
```

----

Questions:

1) Should we change the `with-controller` feature to use `item-controller` (or possibly add another type for `with-controller`)?
2) Does this need feature flagging? (interal change with backwards compatibility)