Support partially downloading videos

[bodqhrohro]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.09.20
• I've searched the bugtracker for similar feature requests including closed ones

Description

I often need to download the whole video only to extract from it a short clip later, with ffmpeg's -ss and -t keys. Thus downloading the rest of the video is redundant, and consumes lots of traffic and disk space.

For HLS videos, which are present on lots of supported sites including YouTube, only certain chunks
may be fetched if their timing is known. Thus the time range may be specified via CLI parameters, either the start point and the duration, or the start and end points. youtube-dl should automatically pick the smallest number of chunks that fit the specified range; optionally it also may pass the parameters to ffmpeg after downloading, so it trims the unnecessary parts from the edge chunks. If the video is not chunked, youtube-dl may fall back to downloading the whole video, or trigger an error.

[antgiant]

Please see #622.