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.
could just be `var silentMode = options.silentMode || false;` what about using `silent` instead of `silentMode`?