Support back-and-forth (yo-yo)

[sindresorhus]

A few people have requested this over the years. I'm still not sure we should do it. There are endless amounts of editing features we could add but I would like to keep Gifski focused on converting and simple. Show your opinion on this with GitHub Reactions.

---

If we were to implement this, we would have to take the video track element, copy it, reverse the copy, and then put the copy after the original.