Descendant property

Fixes an issue where the `jsonRoot` would only reach top level properties instead of descendant properties in a nested json object like this:

```javascript
{
  "parent": {
    "child": {
      "the_root": {
        ...
      }
    }
  }
}
```

Fixed by adding a dot notated string to either the `jsonRoot`, `jsonRootSingle` or `jsonRootMany`.
```javascript
$config: {
  jsonRoot: 'parent.child.the_root'
}
```