Allow passing data-properties to menu items.

I needed to attach data-properties to each menu item (created in a loop) and therefore added this.

Usage:

```
menu: [
  { title: 'item a', data: { id: 123, 'some-other-property': 42, action: ... }
]
```

which adds `data-id="123" data-some-other-property="42"` to the a-element.