Write all item fields as data- attributes

Thinking more about #290, it's not entirely what I need.

I've got a scenario where I have many different object types that will be displayed on the timeline. 

I'd rather not have to update the `dataAttributes` option array each time I add a field, so I added the ability to use the string `'all'` instead of the array which will put every item field as a `data-` attribute.

The old functionality still works.