Fix CMYK from-conversion and add it as possible color function

[Betelgeuse1]

I stumbled across this project yesterday, and it's a really cool tool :)

I'm trying to create a colorscheme for my text editor base on some design book I own, in which all colors are specified using the CMYK format.
Using pastel to convert from CMYK to hex, so my editor would understand it, seemed like a no-brainer, but even though the Self::from(CMYK) function exists, it is never used anywhere in the project.
Also, I've noticed the calculation were wrong in that function, so I fixed those and add the corresponding parse_cmyk function, so it can be used as a color value in commands.

This is my first time "writing" rust code, please feel free to report any mistake I've made :)
I would understand if this is rejected because it's too niche of a usage.

[sharkdp]

but even though the Self::from(CMYK) function exists, it is never used anywhere in the project.

Strange…

Also, I've noticed the calculation were wrong in that function, so I fixed those and add the corresponding parse_cmyk function, so it can be used as a color value in commands.

Thank you!

[sharkdp]

For the other color spaces, we have tests that make sure that we can convert e.g. HSL -> Lab -> HSL and get the same color back. You can search for roundtrip in this file to see some examples. It would be great if we could add tests for CMYK, too.