Add onOptionToggle event to MultiSelect

[TylerWanta]

Currently the MultiSelect always emits the entire list of selected items whenever a single one is selected / deselected. This makes it very inefficient to know which single option was selected / deselected. I'd hate to have to keep a copy of the selected items and then search through the entire list of currently selected items just to find out which one was deselected. It would be easy to emit the single option selected in onOptionSelect, in MultiSelect.vue.

I can create a pull request if it would help