Add some convenience APIs to OnigString

[maxbrunsfeld]

This adds three APIs to the OnigString class:

• .length
• .toString
• .substring

We pass the lines to tokenize around to a ton of different methods in first-mate. This way, we can replace the string with an OnigString without having to also pass the original string around.

I've also converted all the CoffeeScript to JS.

[thomasjo]

Looks like this might be leaking, unless there's some magic going on in node::ObjectWrap? 🚰
Regardless, it can be modernized to using smart pointers to guard against leaks;

std::shared_ptr<OnigString> string = std::make_shared<OnigString>(content);

Personally I also would have used the auto keyword here, but we don't seem to be using that in any of our C++ projects, hence the full type. And note that if the Wrap function does perform magic, smart pointers might not work as intended 🎱

[maxbrunsfeld]

Yeah, I think the ObjectWrap class causes delete to be called in the Javascript object's finalizer.