Enable `data-preview-link` on anchor tags with nested elements

Without this change, `data-preview-link` only works on `<a>`s with no child elements.

e.g.
```html
    <a href="http://foo.bar" data-preview-link="true"><img src="kitten.jpg"></a>
```
does not open the link in a preview iframe because `event.target` is the child `<img>`.