Enable Sound Feedback

For macOS only (for now)

You can now get sound feedback when your tests pass or fail.
Add the --notify flag on your retest command to enable sound. You should hear a sound as your start Retest and on all the runs.

Why? This removes the need to see the terminal window while refactoring code. 🎉

Here are some valid uses of the notify flag:

$ retest --notify
$ retest --all --notify
$ retest --notify 'ruby <changed>'

This release includes:

• #89