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.h

@@ -103,6 +103,8 @@ void decoder_initialized(struct decoder * decoder,
 			 const AudioFormat * audio_format,
 			 float total_time);
 
+enum decoder_command decoder_get_command(struct decoder * decoder);
+
 /**
  * This function is called by the decoder plugin when it has
  * successfully decoded block of input data.