tag: #EXT-X-MEDIA attribute: CHANNELS is not recognized

[megaadam]

CHANNELS should be recognized according to the RFC

All audio EXT-X-MEDIA tags SHOULD have a CHANNELS attribute.

Yes, I am aware of func SetCustomTag() but this is an existing tag and even if I could use that to bend the existing tag handler to include the CHANNELS attribute it would be nice if m3u8 can follow the standard.

There is an old and incomplete (lacks reader, lacks tests) PR for this in #168

Maybe, maybe, we (my team), can finish that job.

[megaadam]

Fixed: #183