Externalize MPV command template (#2948)

* externalise MPVTemplate

* Remove unnecessary comment

---------

Co-authored-by: Deluan <deluan@navidrome.org>

conf/configuration.go

@@ -57,6 +57,7 @@ type configOptions struct {
 	SubsonicArtistParticipations bool
 	FFmpegPath                   string
 	MPVPath                      string
+	MPVCmdTemplate               string
 	CoverArtPriority             string
 	CoverJpegQuality             int
 	ArtistArtPriority            string
@@ -303,6 +304,8 @@ func init() {
 	viper.SetDefault("indexgroups", "A B C D E F G H I J K L M N O P Q R S T U V W X-Z(XYZ) [Unknown]([)")
 	viper.SetDefault("subsonicartistparticipations", false)
 	viper.SetDefault("ffmpegpath", "")
+	viper.SetDefault("mpvcmdtemplate", "mpv --audio-device=%d --no-audio-display --pause %f --input-ipc-server=%s")
+
 	viper.SetDefault("coverartpriority", "cover.*, folder.*, front.*, embedded, external")
 	viper.SetDefault("coverjpegquality", 75)
 	viper.SetDefault("artistartpriority", "artist.*, album/artist.*, external")