Indexes on array properties

For Lens I'd like to access all figure_refs for a given node. So I'd have to tell the index that it should take the nodeid which corresponds to the first element from `path`.

``` js
  "figure_refs_by_node": {
    "type": "figure_reference",
    "properties": ["path[0]"] // no idea how to specify that
  }
```

Desired usage:

``` js
doc.find('figur_refs_by_node', 'paragraph_12');
```
