Easier controller/route overrides

Related to #419, per recommendation from @jferris.

Expands on work done by #416, which allows the developer to disable the default clearance routes.

Adds a generator which does two things:
1. Adds the correct configuration to `config/initializers/clearance.rb` to disable the default routes
2. Injects the default routes into the application's `routes.rb` to allow the developer to make modifications