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 RockstarLang/rockstar#276 gets added to upstream :)