Define a hook to run steps when a UA starts/ends an audio interruption

[youennf]

Define a hook to run steps when a UA starts/ends an audio interruption.
This would be useful for AudioSession as well as Audio WG for AudioContext interrupted state.

[youennf]

@padenot

[padenot]

@gabrielsanbrito, as discussed.

[gabrielsanbrito]

@padenot @youennf I created PR #41 to integrate the changes in WebAudio/web-audio-api#2611 to the Audio Session API spec. I think we can merge #41 after we get one extra +1 on the Web Audio "interrupted" state PR.

Please let me know if this looks good to you.