add support for user defined mpd host, password and port in config file

doc/config

@@ -7,6 +7,12 @@
 #
 ## set it in order to make tag editor and renaming files work properly
 #
+#mpd_host = "localhost"
+#
+#mpd_password = ""
+#
+#mpd_port = "6600"
+#
 #mpd_music_dir = "/var/lib/mpd/music"
 #
 #mpd_connection_timeout = "5"