Add config option to disable automatic mounting of engine

If you have a catch-all route in your rails application (e.g. because you are using pushstate and want to render the same thing on every page and sort it out in JS), you need to explicitly mount engines before the catch-all route.

However, if the engine is auto-mounted, manually mounting the engine causes an error on application boot.

I added a config switch that disables auto mounting so that you can manually mount the engine at the appropriate place in your application's routes.