This uses a new <Compatibility script="..." /> tag in a mod's XML file.
A compatbility layer is a script that runs before mod-init.lua is loaded,
and before any mod scripts are loaded when resuming a saved game.
This is a better solution than shipping a fragile wrapper
with every mod, that tends to break and then needs to be updated.
Now this wrapper is centralized, easy to use, and easy to update.
Closes#31.
