make a mod in another directory

This pull request will bring the mod-ability to Untrusted.

I'm translating Untrusted into Chinese and plan make some funny levels in a Chinese culture background.

However I don't want to replace the source code in Dr. Eval's directory. That's why I need the mod-ability. And I believe it should be useful for other people who would like to make there own story with Untrusted.

And thank you for the great work!