Better nREPL integration

[jeff303]

The Clojure main entrypoint opens the client, and then (essentially) invokes nREPL's main. This isn't really the nicest experience.

• The "welcome" message is very much nREPL specific
• The default prompt of user=> is used before commands
• The editing experience kind of sucks (ex: up arrow just inserts gibberish, and no history), as outlined here