Add support for shebang

[nilscript]

It's not in the spec but it would make my job easier orchestrating scripts without having to wrap them around a maiden call. Example usage after implemented shebang: test.rock:

#!/<PATH>/maiden

Midnight takes your heart and your soul
<rest of lyrics>

[palfrey]

I'd be interested in patches for this, especially if https://github.com/RockstarLang/rockstar/issues/276 gets added to upstream :)