Add support for the allowClear option.

I started using this component a few weeks ago! Thanks so much for creating it.

I needed the ability to access the `allowClear` option on select-2 - this implements it, adds documentation to the docs, and a simple test.

I think it'd be cool if the documentation tracked closer to the select-2 docs themselves. If you're open to the idea, I might take a stab at making them a little bit closer. Let me know what you think!