Possibility for an "open-all" prop

[PJAckerman]

I really like the component, thank you for your work on it.

Would it be possible to add something like an "open-all" prop that will expand all nodes?

[suusan2go]

@PJAckerman like vuetify treeview component's function updateAll ? it looks good! https://vuetifyjs.com/ja/components/treeview/

I don't have much time now, but a pull request is welcome.