Is your feature request related to a problem? Please describe.
Reducing images colors from RGBA to P make big splash of colors and loose some graduality. Being able to have some dithering (event if it make the image bigger because less compressible) would make the resulting quality better.

Describe the solution you'd like
Alllowing to have dithering applied when reducing image colors from RGBA to P.

Describe alternatives you've considered
Doing it myself, but then it goes against the usage of opimize-images. I also tried to add dithering line 135 of img_aux_processing.py without success :/