Add silent mode support to a avoid a stream crash on error

Hello,

  I've tried to use your plugin with gulp-watch and I've found that it could be nice to avoid system.exit(1) every time when you broke your tests. So I inspired myself by ncrunch behavior to propose you such kind of functionality as slient-mode. I hope that you'll find this useful.
The only intention is to avoid an exception to be threw and stop the stream, by the way it must notify user about tests progression in console. No file creation was intended except if user decide to redirect the stdout to a file.