Not all requirements can be expressed in terms of polymorphic url
options so add a `route_for` method that allows calling another
direct route (or regular named route) which a set of arguments, e.g:

    resources :buckets

    direct :recordable do |recording|
      route_for(:bucket, recording.bucket)
    end

    direct :threadable do |threadable|
      route_for(:recordable, threadable.parent)
    end

This maintains the context of the original caller, e.g.

    threadable_path(threadable) # => /buckets/1
    threadable_url(threadable)  # => http://example.com/buckets/1