pulseaudio: Add 'sink-mapping' config option

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"
  }

man/waybar-pulseaudio.5.scd

@@ -113,6 +113,10 @@ Additionally, you can control the volume by scrolling *up* or *down* while the c
 	typeof: array ++
 	Sinks in this list will not be shown as active sink by Waybar. Entries should be the sink's description field.
 
+*sink-mapping*: ++
+	typeof: object ++
+	Sinks named by the values of this mapping will be considered to be the current sink instead of the sinks named by the respective keys.
+
 *menu*: ++
 	typeof: string ++
 	Action that popups the menu.