Consider adding additional admin limitations designed for Authors/Contributors

From what I've read it seems like there is currently no way to add an author or blog contributor to Grav and give him/her posting abilities without also letting him/her be able to edit any and every post/page. This seems to be the only option to create a user who is an 'Author' or 'Contributor'

```
access:
admin:
login: true
super: false
pages: true
site:
login: true
state: enabled
```

But I certainly don't want my guest post authors or once a week posters to be able to edit my home page.