Add section navigation

Our designer @montypivotal came up with a design for our styleguide that included sub-navigation between sections with multiple children, which is great for complicated sections with many children. See the nav pills below the "Typography" heading in the mock below.
 
![pui-16](https://cloud.githubusercontent.com/assets/39398/4511374/15bd36be-4b34-11e4-8223-b683f4c984ab.png)

This PR allows users to include sub nav on top level sections or on all sections via a configuration option `nav_level` in the yml.

We'd love feedback, thanks!
@gpleiss & Nicole