peek: add gifski support

This is an optional feature for gif post-processing.
It produces much better quality gif.

Also added which to PATH with the wrapper, as it's
needed to check for these types of programs [0].

[0]: https://github.com/phw/peek/blob/8b48e8847ad6e1c09fde12972b917bc975153c9c/src/utils.vala#L57