Allow multiple dependent keys for `Ember.computed.filter`

It would be great if `Ember.computed.filter` could take multiple dependent keys. This is an example of how that could be achieved. Some questions:

- Should this be applied to other computed macros too?
- Are zero dependent keys allowed? (regular computed properties allow `.property()`)

(also, if anyone want's to grab this and have a go at it, feel free :)