command: "load" supports remote playlists (m3u, xspf, lastfm://)

This patch integrates the playlist plugin API to the MPD core.  We'll
be able to do much more in the future with that API, that's just the
beginning.

src/command.c

@@ -22,6 +22,7 @@
 #include "playlist.h"
 #include "playlist_print.h"
 #include "playlist_save.h"
+#include "playlist_queue.h"
 #include "queue_print.h"
 #include "ls.h"
 #include "uri.h"
@@ -707,6 +708,10 @@ handle_load(struct client *client, G_GNUC_UNUSED int argc, char *argv[])
 {
 	enum playlist_result result;
 
+	result = playlist_open_into_queue(argv[1], &g_playlist);
+	if (result != PLAYLIST_RESULT_NO_SUCH_LIST)
+		return result;
+
 	result = playlist_load_spl(&g_playlist, argv[1]);
 	return print_playlist_result(client, result);
 }