Added support for col format - used by Aseprite and other software

I've added support for parsing and writing col files. Aseprite is very useful as it supports many formats so I can use it to figure out the inner workings of them.

col is simple binary format where the r, g, and b values of each color are stored one after the other. It's thankfully far simpler than Clip Studio Paint's cls format, it took me about half an hour from start to finish.

As always, let me know if you encounter any bugs.