feat(middleware): add guest option in auth middleware

[ricardogobbosouza]

In most cases you need to create guest middleware:

// middleware/guest.js
export default function({ store, redirect }) {
    if (store.state.auth.loggedIn) {
        return redirect('/');
    }
}

With this feature I can simply add an option:

No need to create a new middleware.

// pages/guest.vue
<template>
  <div>guest</div>
</tempalte>

<script>
export default {
  middleware: ['auth'],
  options: {
    auth: 'guest'
  }
}
</script>

[ricardogobbosouza]

@pi0 @Atinux any chance you could take a look at this PR? Thanks!

[pi0]

Nice feature @ricardogobbosouza :) Thanks!

[pi0]

This PR has been published in v4.6.0