Added optional jQuery element selector useful for intro defined using `steps`

Intro defined using options.steps can benefit from jQuery selector
especially when used in responsive sites where some elements might be
hidden, or in more complicated dynamic layouts.
### Example ###
```javascript
intro.setOptions({
  jQuerySelector: true,
  steps: [
    {
      element: '#title.desktop:visible', // this selector string will be evaluated using jQuery
      intro: "Will display intro item for this element only when it is visible (only desktop)"
    },
  ]
```