Number.prototype.to:  add func parameter

The API wiki says:
> (1).to(15, function (x) { return x * x; }); // ↦ [1, 4, 9, 16, ..., 225]

Yet it doesn't, instead you get [].

This update should still be backwards compatible and allows you to instead do:

> (1).to(15, function (x) { return x * x; }, function (x) { return x + 1; }); // ↦ [1, 4, 9, 16, ..., 225]

It's slightly more work to add the stepper function but then you can get the result the API example says you should get.