Changes to extractCMYKChannel with accompanying example and docs.

[robinsloan]

Hi there! First, as preamble: p5.riso is amazing---totally ingenious, instantly useful. I've already run images generated with this library through my trusty Risograph here in Berkeley.

There's a CMYK channel manipulation technique I learned from Riso wizard Max Stadnik that I've found myself using a ton. Looking at the code for p5.riso, I realized it wouldn't be too difficult to implement… so, I did!

With the Riso, you're often going from a four-channel CMYK image to a two-color Riso print, and it can sometimes---not always, but sometimes!---be useful to "compress" two CMYK channels into each Riso color layer. The results are sometimes more "natural" looking, because you're discarding less color information. The results can also be super weird! Either way, it's just a nice option to have.

The example I've included in this pull request is probably a more eloquent explanation than this.

Thanks again for a great library!

[antiboredom]

Thanks so much for the kind words - we're really glad you're liking the library and using it!!

And, thanks for this wonderful addition. I wasn't familiar with the technique and now really excited to try it out.

Do you have any example prints using CMYK compression? If so, we could add one to the documentation page.

Thanks again!

[robinsloan]

Yes indeed! Here's one where the original was a painting of a normal-looking (beautiful) ocean scene. Its MY channels went to fluorescent orange, CK to dark teal:

[antiboredom]

amazing - thanks! Just added it to the docs page.