encoder: added vorbis encoder

This new vorbis encoder plugin is based on the existing shout_ogg.c
plugin.

src/encoder_list.c

@@ -22,7 +22,12 @@
 
 #include <string.h>
 
+extern const struct encoder_plugin vorbis_encoder_plugin;
+
 static const struct encoder_plugin *encoder_plugins[] = {
+#ifdef ENABLE_VORBIS_ENCODER
+	&vorbis_encoder_plugin,
+#endif
 	NULL
 };