Allow basic filtering.

[andrewculver]

Examples I think should be valid:

editor:
  models:
    project:
      - archive
      - where:
        - archived_at: nil

Would be like: can :archive, Project, [all the other magic], archived_at: nil

editor:
  models:
    project:
      - archive
      - where:
        - workspace:
          - archived_at: nil

Would be like: can :archive, Project, [all the other magic], workspace: {archived_at: nil}