[downloader/mhtml] Add new downloader (#343)

[downloader/mhtml] Add new downloader (#343)

This downloader is intended to be used for streams that consist of a
timed sequence of stand-alone images, such as slideshows or thumbnail
streams

This can be used for implementing:

https://github.com/ytdl-org/youtube-dl/issues/4974#issue-58006762
https://github.com/ytdl-org/youtube-dl/issues/4540#issuecomment-69574231
https://github.com/ytdl-org/youtube-dl/pull/11185#issuecomment-335554239

https://github.com/ytdl-org/youtube-dl/issues/9868
https://github.com/ytdl-org/youtube-dl/pull/14951

Authored by: fstirlitz