Fix the --url option

This option appears to be intended to allow the Rack app to be mounted at a specific path, rather than `/`, but it doesn't actually do anything. The `:Path` option is set and passed along to the `Rack::Handler`, but none of the [built-in handlers] (https://github.com/rack/rack/tree/master/lib/rack/handler) appear to support it.

So this patch fixes it by wrapping the Shotgun::Loader app in a `mount` block, which defaults to "/", but can be set with the `--url` option. I didn't add any tests, since all of this stuff happens in bin/shotgun.