Added option prioritize_current_selection.

Currently, the previously selected value is still selected when the user starts new search. With this option (`prioritize_current_selection`), the selection is reset, i.e. the behavior is the same as at the initial search. 

The current behavior would be the default (`prioritize_current_selection: true`).

![chosen-prioritize_current_selection](https://cloud.githubusercontent.com/assets/4896806/2655354/cf615a04-bfe4-11e3-99ea-34a69759c432.png)