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"
  }

src/modules/pulseaudio.cpp

@@ -7,6 +7,7 @@ waybar::modules::Pulseaudio::Pulseaudio(const std::string &id, const Json::Value
 
   backend = util::AudioBackend::getInstance([this] { this->dp.emit(); });
   backend->setIgnoredSinks(config_["ignored-sinks"]);
+  backend->setSinkMapping(config_["sink-mapping"]);
 }
 
 bool waybar::modules::Pulseaudio::handleScroll(GdkEventScroll *e) {