Add ARTISTS= vorbis comment support to libtags

This is mostly just for flac handling, since we use libogg directly for
ogg containers, and flac+ogg are the only containers that use vorbis
comments.

lib/libtags/tags.h

@@ -10,6 +10,7 @@ enum
 {
 	Tunknown = -1,
 	Tartist,
+	Tmultiartists,
 	Talbumartist,
 	Talbum,
 	Ttitle,

lib/libtags/vorbis.c

@@ -11,6 +11,7 @@ static const struct {
 	{"album", Talbum},
 	{"title", Ttitle},
 	{"artist", Tartist},
+	{"artists", Tmultiartists},
 	{"albumartist", Talbumartist},
 	{"tracknumber", Ttrack},
 	{"date", Tdate},