Add an option to set a stepId that will be used on the step's popover div

This very small changes add an option on the step : 

```
tour.addStep({
  element: "",
  stepId: "myPopoverId"
  title: "", /* title of the popover */
  content: "" /* content of the popover */
});
```

The `stepId`, if provided, is used as the id of the popover div element, allowing to apply easily  any custom style.