[youku] Segments instead of m3u8

[Romern]

Checklist

• [x] I'm reporting a feature request
• [x] I've verified that I'm running youtube-dl version 2020.03.24
• [x] I've searched the bugtracker for similar feature requests including closed ones

Description

Currently, the youku extractor only consideres the m3u8 to download the videos, but all videos have segmented mp4 files already included in the json in stream.segs.cdn_url. The m3u8's may have missing keyframes (this was for me the case), so it would be nice to include this method as well, but I am not sure how to edit the module to download all segments.

[zackmark29]

Yeah I need this feature as well.