Add option to clear typeahead on select option

[BrodrickChilds]

Similar to #108 but a bit more basic.

We're using the typeahead component as a part of a custom multiselect form, where we don't want the values to appear as tokens but rather in another place in the form where you can interact with them. Anyways, this allows a straightforward way to clear the input after selecting an option.

Thanks!

[aaroncrawford]

For anyone that needs this in the future, another issue references how to do it with refs. Not documented, so hope this helps: https://github.com/fmoo/react-typeahead/issues/254