tag: added support for MusicBrainz tags

Added all important id tags from the MusicBrainz wiki:

 http://musicbrainz.org/doc/MusicBrainzTag

This should automatically enable its suport in the vorbis and flac
decoder plugins.

src/tag.c

@@ -53,7 +53,13 @@ const char *mpdTagItemKeys[TAG_NUM_OF_ITEM_TYPES] = {
 	"Composer",
 	"Performer",
 	"Comment",
-	"Disc"
+	"Disc",
+
+	/* MusicBrainz tags from http://musicbrainz.org/doc/MusicBrainzTag */
+	[TAG_MUSICBRAINZ_ARTISTID] = "MUSICBRAINZ_ARTISTID",
+	[TAG_MUSICBRAINZ_ALBUMID] = "MUSICBRAINZ_ALBUMID",
+	[TAG_MUSICBRAINZ_ALBUMARTISTID] = "MUSICBRAINZ_ALBUMARTISTID",
+	[TAG_MUSICBRAINZ_TRACKID] = "MUSICBRAINZ_TACKID",
 };
 
 int8_t ignoreTagItems[TAG_NUM_OF_ITEM_TYPES];

src/tag.h

@@ -39,6 +39,12 @@ enum tag_type {
 	TAG_ITEM_PERFORMER,
 	TAG_ITEM_COMMENT,
 	TAG_ITEM_DISC,
+
+	TAG_MUSICBRAINZ_ARTISTID,
+	TAG_MUSICBRAINZ_ALBUMID,
+	TAG_MUSICBRAINZ_ALBUMARTISTID,
+	TAG_MUSICBRAINZ_TRACKID,
+
 	TAG_NUM_OF_ITEM_TYPES
 };