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.
Okay, so we should take the regex from `stripFormatting` and compile it to a `Pattern` for speed, then use `pattern.matcher(unformatted).replaceAll(ChatColor.COLOR_CODE + "\\1")`.