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 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/-u option. I didn't add any tests, since all of this stuff happens in bin/shotgun.
This way you can also pass a full path to the rackup file.
In this version of ruby instance_eval yields self and the default @config is lambda with no arguments which gives ArgumentError.