Feature/link type

The comments in the commits provide the details for this PR, but the basic goal was to support the following API for sections:

```coffeescript
sections: [
  id: 1
  name: 'Home'
  links:
    children: [2, 3]
,
  id: 2
  name: 'Child 1'
,
  id: 3
  name: 'Child 2'
]
```