[FEATURE beta] Allow CollectionView createChildView override to prevent item from rendering

The docs specifically point out the ability to override `createChildView` for more advanced management of each item view. However, given the use cases of the override, I would also assume I should be able to fully omit a child view if it doesn't match my criteria.

My pull request allows this to be possible:

```javascript
CustomCollectionView = Ember.CollectionView.extend({
  createChildView: function(viewClass, attrs) {
    if (attrs.content.kind == 'album') {
      viewClass = App.AlbumView;
    } else {
      return; //I don't want to see songs in this iteration of the view
    }
    return this._super(viewClass, attrs);
  }
});
```