Add :host as an option to 'lein repl :start' and 'lein repl :headless' commands

This change allows the host to be specified on the command line for `repl :start` and `repl :headless` commands, falling back to the host given in :repl-options otherwise, or finally defaulting to 127.0.0.1.

The nrepl start message has also been modified to include the host, i.e.

    > nREPL server started on port 51108 on host 127.0.0.1

This keeps "started on port (\d+)", so that none of the existing tools that search for that pattern break (see discussion on issue #1217), but makes the host explicit.