Add support for swapping out datasets and options at runtime

In my project, I need to be able to swap out datasets for different search types.

Arbitrary example: http://embed.plnkr.co/M0kxQJAa14Xs7eyoTMPW/preview

Let me know what you think. Ideally I would like to be able to keep the dropdown open when they change search types - but for the life of me I can't get typeahead to behave. Have tried `element.typeahead('val', value)`, but that doesn't seem to fly.