Add `ChatColor` enum

This a general class (perhaps better referred to as an `enum`) that handles formatting and colors. Documentation is convoluted and a bit sparse, but I think it's sufficient for now.

Note: I created a new package `org.spongepowered.api.chat` for it, because I wasn't sure where to put the file.
I believe Bukkit didn't do regex/string.replace because it validated the color code before changing the prefix.