Add "live_since" timestamp json key for live streams

[nurupo]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2020.01.15
• I've searched the bugtracker for similar feature requests including closed ones

Description

YouTube and Twitch live streams provide timestamps of when the ongoing live stream has started. It would be useful to have youtube-dl print those in its json output, e.g. so that you could see in mpv (which uses youtube-dl) how long a stream has been live for. So far youtube-dl provides only year, month and day date, but no exact time.

[dstftw]

1. timestamp.
2. Youtube does not provide timestamps.

[nurupo]

The "Started streaming N minutes ago" that you see on YouTube live streams is not exposed in YouTube's API?

[dstftw]

1. That's not a timestamp and it's not necessarily provided.
2. youtube-dl has nothing to do with Youtube API.