Dividers in Navbar defined in the TOC file

It would be really helpful if you could define a divider in the TOS file, to separate out similar links.

This would look something like:
```
- name: Item 1
href: item1/
- name: Item 2
href: item2/
- divider
- name: Item 3
href: item3/
```

### Example use case:
separating off-site links from documentation links.

You can kind of fake it right now with an item like:
```
- name: " "
```
however, this inserts a link, the vertical height is wrong, and it's possibly too wide