pulseaudio: Add 'sink-mapping' config option

[Ape]

This commit adds a new optional config to the pulseaudio modules. It maps sink names to other sink names so that if the current sink is a key, the sink named by the value is considered to be the current sink instead of it.

E.g.

"sink-mapping": {
  "easyeffects_sink": "speakers_sink"
}

[Ape]

Any comments on this?

I have been using this for 2 weeks and it seems to work perfectly for me.

[Ape]

Rebased on the current master. The clang-format failure is not related to this commit.