Add WITH_EXTENDED_VORBIS_FILEINFO knob.
This patch includes information about the average bitrate of the file
(because vorbis uses variable bit rate encoding by default and nominal
might or might not match the actual effective bitrate) as well as
information about the encoder which created the file.
Submitted by: Thomas E. Zander <firstname.lastname@example.org>
Reviewed by: Espen Skoglund <email@example.com>