Also extract metadata from streams

Some formats have metadata in STREAM blocks (eg. sometimes an Ogg Vorbis file
has no metadata in the Ogg container, but the Vorbis stream has some).
This change extacts metadata from streams and adds them as "streams" in
the returned metadata.  It does not overwrite metadata from the FORMAT block,
so that users can decide if they prefer format metadata or stream metadata
when both are present.