Allow configuration of a root path

Now Clearance forces redirects to root path (`/`) everywhere.
It's fine to redefine methods like `url_after_create` as they customise application behavior and you may (for some reason) have many inherited controllers.
But I don't think it's okay to redefine root path everywhere, so I propose a configuration option for that. It still defaults to `/`, so this change should not affect existing apps.