Introduce children key for menu config

Currently Hugo supports father/child relationships trough a parent key on menu items.

It would be nice to also set the relationship from the parent menu item. I suggest the following:

- name: About us
  url: /about-us/
  weight: 0
    - name: Team
      url: /about-us/team
      weight: 1
    - name: Company
      url: /about-us/company
      weight: 2
    - name: Blog
      url: /blog
      weight: 3

This feature opens nicer editing experiences in an CMS like NetlifyCMS. Where you would be able to create a better visual understanding of the menu structure.