Add a setting to disable all routes

This setting allows clearance routes to be completely customized. Users who
customize the routes to this level may have to edit the views and mailers as
there are references to routes there.
Add generator to copy default routes to the app

The generator configures clearance to turn off its internal routes and
then copies the default routes to the host application's routes file
where they can be customized to the developer's content.
Update README with route override instructions