lein run starts an app server, same as a service server

This enables starting a server with `lein run`, same as we do for [service](https://github.com/pedestal/pedestal/blob/master/service-template/src/leiningen/new/pedestal_service/project.clj#L26). While a developer often needs the repl, an end user of an app does not. We should make it just as easy for end users to try a pedestal-app app as we do in service.