Option to automatically kill nailguns before exiting

Optionally (via config or cmdline flag) kill nailguns after goal completion before exiting, for those that prefer pants leave the system as close to the way it found it as possible, without giving up the benefits of nailguns all together during compilation. Running in this mode, the overhead of starting nailguns is incurred once per run, which depending on compile times and nailgun memory usage may or may not be worth while.