Gracefully handle if the port is taken

[jo-sm]

At the moment, if you have two instances of cljest running, such as when you're working on two projects at the same time, and they both use the default (or same) port, the second instance will fail when starting up because it tries to use the same port as the first. We can gracefully handle this, most likely by exposing the entire config from the server via an endpoint. This would have the added benefit of removing some duplicated business logic across Jest and cljest code.