Rewriting the options handling

* Moving :items to options
* Simplifying the Item options handling
* Simplifying item options handling in ItemContainer
* Stop removing :if/:unless options in ItemContainer
* Stop removing container related options in ItemContainer

Breaking Changes

Previously `items` were passed as a separate argument, they are now passed as
the `:items` option.

`ItemContainer` and `Item` used to remove options from the `options` hash
along the way. It's not the case anymore. The `options` hash is not
modified in any way.