Better gif quality support

Currently, `manim` supports creating gifs with the `-i` flag using ffmpeg. I have noticed the output to be of very poor quality, even when rendered on high quality videos. I have managed to make much better quality gifs using https://gif.ski/ which has a cli interface available.

This would be an extra project dependency, albeit only for those who want it. There could even be an additional `--high-quality-gif` flag for users who want this. Any interest in adding this feature?