Add loadingView option to CollectionView

When a collection is fetching elements you might want to indicate
that. To do it you can specify `loadingView` attribute on your collection view.

```js
var LoadingView = Backbone.Marionette.ItemView.extend({
  template: "#show-spinner-template"
});

Backbone.Marionette.CollectionView.extend({
  // ...

  loadingView: LoadingView
});
```