Smarter default port

[Jim Crossley]

Howdy. Recently Immutant adopted Leiningen's convention of writing the nrepl server's port to target/repl-port, particularly handy when the specified port is 0.

Anyhoo, I have the nrepl function a bit smarter about the default port prompted when in a project directory and that file exists, otherwise falling back to using the nrepl-port custom variable. I like it for either connecting to Immutant's repl or Leiningen's if started using 'lein repl' instead of nrepl-jack-in.

If that sounds reasonable, I'm happy to submit a PR.

Thanks,

Jim