ReplayGain support + audio normalization (web player) (#1988)

* ReplayGain support

- extract ReplayGain tags from files, expose via native api
- use metadata to normalize audio in web player

* make pre-push happy

* remove unnecessary prints

* remove another unnecessary print

* add tooltips, see metadata

* address comments, use settings instead

* remove console.log

* use better language for gain modes

conf/configuration.go

@@ -63,6 +63,7 @@ type configOptions struct {
 	ReverseProxyUserHeader string
 	ReverseProxyWhitelist  string
 	Prometheus             prometheusOptions
+	EnableReplayGain       bool
 
 	Scanner scannerOptions
 
@@ -265,6 +266,8 @@ func init() {
 	viper.SetDefault("prometheus.enabled", false)
 	viper.SetDefault("prometheus.metricspath", "/metrics")
 
+	viper.SetDefault("enablereplaygain", false)
+
 	viper.SetDefault("scanner.extractor", consts.DefaultScannerExtractor)
 	viper.SetDefault("scanner.genreseparators", ";/,")