add ability to make a timelapse of recorded video

[allegfede]

I plan to use the yi camera to make timelapses of my ender3 printing.
Get a still every N seconds (configurable in the interface) and add it like the last frame of a mjpeg clip, stored to the sd, would be great.
No need to run a separate pc/tablet to capture a still and recombine them.
PS: resulting clip could be downloaded from the same interface tab where you make the timelapse settings, you start and stop it (or got from the microsd)