Add dithering options

[gingerbeardman]

Is your feature request related to a problem? Please describe.

• currently I have to use custom to do dithering of the image

the order of processing needs to be

1. black & white
2. scaling
3. dithering, eg. -ordered-dither o2x2

Describe the solution you'd like

• new option for dithering

Describe alternatives you've considered

• custom

Additional context

• https://legacy.imagemagick.org/Usage/quantize/#ordered-dither

[TeaM-TL]

I don't want to add more and more conversion method, because it will is way to implement every options of ImageMagick. I think, that the best way is use Custom box as you use now. I have an idea to save favorite custom commands, and use it from list. It means you can store your favorite commands and use it everytime from history