Can't start the same intro.js object twice

The culprit is line 65:

```currentItem.element = document.querySelector(currentItem.element);```

The element selector is overwritten with the actual DOM element. So the next time #start() is called on the same intro.js object, the query selector crashes.

The simplest solution would be to rename 'element' to 'selector':
```currentItem.element = document.querySelector(currentItem.selector);```

The downside is that it breaks the programatic API.