Add UnrealScript support

This is an initial version of the pull request. It's valid, but a bit suboptimal, since right now it uses the JavaScript lexer. UnrealScript is similar to JavaScript, but it's not the same, and many keywords are missing, so I'd like to add a dedicated UnrealScript lexer.

However, I could find no information at all about how to do that. Could someone point me in the right direction? That would also be generally useful information, so it should be documented somewhere. Maybe this project could have a wiki attached to it for adding information like that?