Add support for lyrics.

Not supported in id3v1 and id3v22

tag.go

@@ -87,9 +87,12 @@ type Metadata interface {
 	// Disc returns the disc number and total discs, or zero values if unavailable.
 	Disc() (int, int)
 
-	// Picture returns a picture, or nil if not avilable.
+	// Picture returns a picture, or nil if not available.
 	Picture() *Picture
 
+	// Lyrics returns the lyrics, or an empty string if unavailable.
+	Lyrics() string
+
 	// Raw returns the raw mapping of retrieved tag names and associated values.
 	// NB: tag/atom names are not standardised between formats.
 	Raw() map[string]interface{}