Add #dom_attributes option to ItemContainer.

This allows you to define other html attributes than :id and :class on menu container. I needed this in order to define some 'ng-...' attributes on `<ul>` element, which was impossible for me without this change.

This includes backward compability for `item_container.dom_id=` and `item_container.dom_class=`, so the old way of setting it is still okay and won't be broken.