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.
Something like `return unformatted.replace(alternateCode, ChatColor.COLOR_CODE)` would be better, because there's no need to regex a simple replace.