Feature request. Generics for externs

It is currently possible to use std::vector (in a little bit complicated way), but no ways to use std::array (or I just don't know how).

So would be nice to have something like:
```
@:native('std::array') // I think it is hard to put something like this,
@:native('std::array<{0}, {1}>', T, S) // so maybe it may be as this?
@:native('std::array') // or like this and T and S will be atomatically added as template values
extern class StdArray {
...
}

//and somewhere in code:
var arr:StdArray = ....;
```

This may help a lot to work with C++ templates in Haxe.