Add an option to make a certain percentage of sheep named jeb_ for rainbow and allow us to customize the chances for each normal natural sheep color to spawn

[BrailleBennett]

Currently, there are no options to customize sheep spawning. Since Purpur already has options to name a certain percentage of rabbits toast on spawn or name a certain percent of vindicators Johnny, having a chance for a natural rainbow sheep would make sense as a natural continuation of that. Additionally, I think it would be cool if we could customize the chance that a given spawned sheep is pink, etc. To make them rarer or more common.