[v3] Add onBeforeStackUpdate prop

[christianchown]

Hi Sergey - can v3 match the functionality of v2 and add an onBeforeStackUpdate prop?

<Navigator onBeforeStackUpdate={
  (stack, nextStack, transitionProps) => 
    console.log(stack, nextStack, transitionProps)
} />

Use case: being able to synchronise state changes of components with router updates e.g. tabs