Use the contents of target/repl-port as default port, if present

Both Leiningen and Immutant write the port for the nREPL server they start to `target/repl-file`. This change uses the contents of that file as the default value when the `nrepl` function prompts for the port. If the user is not in a project directory or the file is missing, the custom variable, `nrepl-port` is used as the default value, just as before.