demux_mkv: Support playing Opus streams in Matroska

FFmpeg recently changed how it writes Opus-in-Matroska to match
the A_OPUS/EXPERIMENTAL name that mkvmerge uses, with the caveat
that things will change and compatibility with old files can get
worked out when the spec is finalized.

This adds both A_OPUS and A_OPUS/EXPERIMENTAL so that *hopefully*
it can play both the newer files that use A_OPUS/EXPERIMENTAL, and
older ones muxed by FFmpeg that were simply A_OPUS, since this is
also what FFmpeg seems to be doing to handle the situation.

The only thing I'm [highly] unsure of is the private data stuff, which doesn't do a whole lot at all, just enough to make sure it can play the files.