Move lib/cli.js to bin/cli.js, extracting argument parsing back into lib/args.js
Add tests for lib/args.js
Move lib/cluster.js -> lib/cluster/index.js
Remove unused private function.
Move defaults objects into lib/args.js
Extract lib/cluster/worker.js from cluster/index.js
Add stub tests for cluster master and worker.
Extract lib/defaults.js out of lib/args.js
Only call worker#listen() if cluster.isWorker
Change port default to 8000, matching app.js
Add --cluster option to CLI, defaulting to single-threaded default server.