Allow easy overriding of clearance controllers

This is particularly useful for overriding the controller used in the sign_in,
sign_up, and sign_out routes. Rails 4 no longer allows overriding named routes
(an exception is raised), so having `sign_in` use your customized controller
(even when it was named `users`) required adding an unnamed (`as: nil`) route
that matched the same path and went to a different controller.

This difficulty is not a problem for resourceful routes. That is, adding your
own `resoures :passwords` to your routes file will override the routes as
expected. However, configuration was added for all controllers for the sake of
consistency and because it allows the user to change the controller without
having to know which actions the resources need to have available for
Clearance.