[2894] Allow initial render transitions

Would it be a pain in the ass to allow initial render transitions in the transitions object?

https://vuejs.org/v2/guide/transitions.html#Transitions-on-Initial-Render

Example:

module.exports = {
  transition: {
    name: 'page',
    mode: 'out-in',
    appear: true,
  }
}

<transition name='page' mode='out-in' appear>
  <!-- ... -->
</transition>