Add new option to allow the currently selected value to be displayed when the searchbox opens up

First of all thank you for this project, it is well written and really useful :)

I have added an option called "keepValOnSearch" that when set to true will displays the currently selected value inside the searchbox. It allows to user to easily copy/paste or edit it (instead of rewriting it). 

This new option works only for the single select without ajax calls. 

Let me know if this is of any use or if some adjustment must be made to my changes.