Implement a |changed filter plugin

As proposed on list at http://goo.gl/ECS4xm

I'm not sure what Michael meant with _The only difference is the filter understands lists when there is not a top level .changed element._ Is this about registered loop tasks (with_items) where different items yield changed or not? AIFACS, the main result already sets `changed: true' when at least one of the item subtasks had a `changed: true`.

Either way, I gave that a try in the next commit. Let me know what exactly is expected.