Support defining itemViewContainer in CompositeView instance creation

This allows to set the `itemViewContainer` in the CompositeView creation, not just in the View definition. This goes in sync with how the other parameters behave in these two scenarios.

So, you can either define the itemViewContainer like this:

```js
MyCompositeView = Backbone.Marionette.CompositeView.extend({
  itemViewContainer: selectorOrFunction,
  ...
});
```
or like this:
```js
var view = new Backbone.Marionette.CompositeView({ //or a custom view that inherits from CompositeView
  collection: someCollection,
  model: someModel,
  itemViewContainer: selectorOrFunction
});
```