Capture desktop video script?

After seeing @kieranjol make a comment on a request for recording software, I saw that this can be done via ffmpeg pretty easily and I think it'd be helpful to have this script added in.

It might be a bit of a challenge due to the multiple operating systems? https://trac.ffmpeg.org/wiki/Capture/Desktop

Even if we added just one and referenced this perfectly-good-wiki-entry, I think that might add value because it showcases that something is possible for people who might not know that it is possible.