Add --ext_video argument, default to auto

[Splendide-Imaginarius]

This enables output container formats such as .mkv, .avi, .webm, and .gif.

As far as I can tell, the --ext argument isn't actually used anywhere. I assume that's a bug and that it's intended to be used only for images (as its help text says), so I created a separate argument for videos rather than using --ext for this. I'm open to changing this if requested.