Adding Response#add_audio_stop

[charlotte-miller]

Thank you for sharing this gem; it's been clear and straight-forward. Using the gem, I quickly had audio playing from my test device. But... I had no way to stop it (w/out leaving the app).

This PR adds a method to stop audio. An example:

case input.name
when 'AMAZON.PauseIntent' then output.add_audio_stop
...

Let me know what you think.

Source: Amazon docs