Allow element to be type function that returns an element

[richie-south]

Adding option to use element as function that returns an element. This makes it possible to have steps that selects elements that might not on be selectable right now but will be later on.

usage:

const driverObj = driver({
  animate: true,
  steps: [
    {
      element: () => document.querySelector(".page-header"),
      popover: {
        title: "",
        description:"",
      },
    }
  ]
});

driverObj.drive();