[Feature request] Syncable focus prop

It would be convenient to have a bound focus boolean prop with an update:focus event.

update:focus would emit with a boolean every time vue-multiselect is focused or blurred.

The parent component can then also change the focus prop to cause vue-multiselect to activate/deactivate on demand.

This would eliminate the need to get a ref of the vue-multiselect component to call the activate()/deactivate() methods on it since that could just be handled through the focus prop.

Issue #445 will also be addressed with this feature because you could just put focus="true" on vue-multiselect to have it always remain open.