Support for connecting to nREPL server exposed over unix domain sockets

[mk]

nREPL added support for listening on unix domain sockets in 1.0 which have seen first-class support in the JDK with JEP-380.

I think having this could offer nice usability, especially when dealing with multiple REPLs since we could use nonconflicting names in different projects. Maybe we could also establish a naming convention on how to name the socket files e.g. repl-name.nrepl-socket which an editor could look for similar to what's currently happening with .nrepl-port.