Create Announcer action

Reintroduces a simplified "Announcer" action:

- Activating whilst in `standby` state announces "Now in standby"
- Activating whilst in `online` state announces "Now playing <name of station>"

The `stopAnnouncing` action is triggered when the track finishes playing. It can also be cancelled mid-way through playback.
Could you please use the `logger` object instead?