Add CMYK output support

[elmimmo]

Is your feature request related to a problem? Please describe.
Plenty of comics intended for print are in the CMYK color model. Furthermore, in plenty of cases (but not necessarily), text will only exist in the Black channel.

Describe the solution you'd like
It'd be nice if Panel Cleaner could process CMYK images and output them as such. As of now, it seemingly converts them to RGB.

Describe alternatives you've considered
If something in the pipeline is not ready to process composited CMYK images, then please explore processing each channel separately and combining them al a latter step.

[VoxelCubes]

I'll see what I can do. The Pillow library should be able to support it.

[VoxelCubes]

I've got some free time coming up, but to make sure this actually works, I'd need some proper cmyk samples. Please send some to voxel.aur@gmail.com to make sure I get this right.

[VoxelCubes]

Thanks, I got it. That'll help a lot. In principle, the processing happens on an RGB copy, because that's what the AI expects, but the final output is composited on the original file, so hopefully I can ensure the special channel configuration stays preserved.