Add option for merging opus audio into mp4 containers

[peng-555]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2019.11.22
• I've searched the bugtracker for similar feature requests including closed ones

Description

Chrome and Firefox has supported mp4 containers with opus audio since version 70 and 63 respectively. The videos play natively in the browser.

The support for mp4+opus in chrome is detailed here: https://www.chromestatus.com/feature/5100845653819392

For ffmpeg to merge opus in mp4 containers the -strict -2 option needs to be used in ffmpeg

I've testing thse videos in Firefox, Chrome, mpv, VLC, & Totem and all play correctly

[a67878813]

I have the same request.

[no-boot-device]

Having an option to replace "-c copy" with "-vcodec copy" so that opus is properly encoded to aac without the speed loss of re-encoding the video would be nice too.

[tripulse]

OPUS codec is experimental for MP4 isn't it? I’m not claiming it, FFmpeg says so.