[Feature Request] natively support downloading specific time frames

[blackjack4494]

• [ ] I'm reporting a feature request
• [ ] I've verified that I'm running youtube-dlc version 2020.09.14
• [ ] I've searched the bugtracker for similar feature requests including closed ones

WRITE DESCRIPTION HERE In the past I also needed specific time frames. Lately some people were looking for such feature as well. There is a workaround using ffmpeg.
I have no clue yet how to name those flags but to show what I mean
--start-time 30s where also 1m 1hr 1d should work as well as 1m30s. This flag should only download media after e.g. 30 seconds (lower threshold).
--duration 30s same parameter behaviour as above. This flag should only download 30 seconds of media either starting from 0 (default) or if start time is giving take that.
--end-time 50s again same parameter behaviour. This flag should download no more than the given time (upper threshold). Either always enforce this when conflicting with duration flag or make it depending on argument position e.g. if duration comes after make duration override end time if needed.

Benefits would be to have this feature natively without the need of ffmpeg as external downloader.