Get closest color if TrueColor not supported

[spenserblack]

This PR makes TrueColor convert into one of the more "basic" colors (Color::Black, Color::Red, etc.) if TrueColor is not supported based on the COLORTERM environment variable. It gets the closest color based on Euclidean distance.

I'm not sure if some of the functions I added should be made public or not, so I left them private for now.