Add piggieback support to repl-launch

https://github.com/cemerick/piggieback

Piggieback allows launching a ClojureScript REPL over an nREPL session. nREPL is the favored channel for many Clojure dev environments, and offers better support than the current REPL launched, for example, by `lein trampoline cljsbuild repl-launch phantom phantom/repl.js http://localhost:3000`, which simply runs in the terminal.