Generate nav links instead of hard coded in template

We should consider adding a feature that generates the links in the main nav, rather than being hard coded.

As I have been creating multiple themes for DocJS, I've noticed that it's a pain to switch themes on a project. It's not possible to just update my `documentjs.json` file with the new theme because my main nav links are hard coded. So then I update the `layout.mustache` and now I've modified the theme. That makes it more difficult to share themes across projects or update them.
### Suggestion

Allow defining nav links in a markdown file, much like pages:

```
@nav Home index.html 1
@nav Components components.less.html 2
```

These would then be generated using a new template, like `nav.mustache`

```

{{#eachNavItem item}}

• {{title}}

{{/eachNavItem}}

`nav.mustache` is included in `layout.mustache`