Add Host as arg to "repl :start" and "repl :headless" commands.

This is a step towards easier setting up of remote nrepl
sessions, see issue #1217.
Include host in nrepl start message.

This makes the default use of localhost for nrepl port binding
explicit, see issue #1217.
Merge pull request #1223 from kanej/hostreplarg

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