plugins.douyutv: Add support for transcodes.
stream.hls: Rename namekey and nameprefix to name_key and name_prefix.
stream.hls: Add check_streams option to parse_variant_playlist().

When enabled we check that the sub stream is available before
adding it.
plugins.oldlivestream: Make use of the check_streams option.

Resolves #560.
Update CHANGELOG.rst.