Allow addition of custom tooltip styles via data-tooltip-style

To fix some quirks and add a degree of customization, I've added the ability to set the style of a tooltip for a particular element using a `data-tooltip-style` attribute:

```html
<div data-intro="Hello!" data-step="1" data-tooltip-style="right: 0px;width: 88%;"></div>
```

A similar addition could be made to allow customization of the helper layer.