tag/Config: support incremental "metadata_to_use" setting

With a "+" or "-" prefix, the "metadata_to_use" setting can manipulate
the existing default.  This way, one can enable `TAG_COMMENT` without
having to list all the other tags.

doc/mpdconf.example

@@ -111,6 +111,10 @@
 # found in the user manual.
 #metadata_to_use	"artist,album,title,track,name,genre,date,composer,performer,disc"
 #
+# This example just enables the "comment" tag without disabling all
+# the other supported tags:
+#metadata_to_use "+comment"
+#
 # This setting enables automatic update of MPD's database when files in 
 # music_directory are changed.
 #