Vuex Compatible

In previous releases the only way to sync up the selected value with a parent was to use two-way data binding. This clashes with Vuex, so I've added a new callback prop onChange.

• the value prop is no longer required, and does not need to use a two-way binding. Two way binding can of course still be used to sync the value to a parent, but it's not required.
• onChange is a new callback function prop that receives the updated value any time it changes. Useful for integrating vue-select with Vuex.