added decoder_get_command()

Another big patch which hides internal mpd APIs from decoder plugins:
decoder plugins regularly poll dc->command; expose it with a
decoder_api.h function.

src/decoder_api.c

@@ -45,6 +45,11 @@ void decoder_initialized(struct decoder * decoder,
 	notify_signal(&pc.notify);
 }
 
+enum decoder_command decoder_get_command(mpd_unused struct decoder * decoder)
+{
+	return dc.command;
+}
+
 /**
  * All chunks are full of decoded data; wait for the player to free
  * one.