Endpoint for fields

Hi @eduardoboucas

First of all thanks for your great tool.

By reading the README I saw the `sort` parameter is able to sort entries by a field (that is inside the data files). All of this is very well but I was worried about a point.

Tell me if it is just a question or if it is a feature request: I wanted to know if it is possible to generate endpoint for fields (that are inside data files).

To keep you example you are able to generate endpoint for `language`, `genre`, `year`, `movie`. But if data files (yaml or JSON) are containing a structure like:

```json
{
"budget": 10000000,
"directors": [...],
"writers": [...],
"stars": [...]
}
```

Would it be possible to generate endpoints for `budget`, `directors`, `writers`, `stars` ?