Add pingpong mode to giftool command. When --pingpong flag is used,
the generated GIF will play forward then backward, creating a
back-and-forth looping effect.
Features:
- Add --pingpong option to giftool command
- Support combining with --speed for adjusted playback speed
- Update documentation with new option
Examples:
- giftool [图片] --pingpong
- giftool [图片] --pingpong --speed 2.0
