Add new function `goToStepNumber`

This allows to go to a specific step with the specific `data-step`
attribute. This differs from `goToStep` in the way that `data-step`
does not have be continuous to pick the desired element.

For example:

```
<div id="first" data-step='5'></div>
<div id="second" data-step='9'></div>

// to get #second one can do now:
introJs().goToStepNumber(9); --> #second

// or with goToStep:
introJs().goToStep(2); --> #second
```