Parsing nested fields of object

Thank you for this awesome project!

It would be great if this library had some way to parse nested fields with ignoring others.

For example

```
{
  "foo": 123,
  "bar": {
    "targetField": [
      {"a": 1},
      {"a": 1},
      {"a": 1}
    ]
  }
}
```
```
tokens.through(TokenFilter.downField("bar").downObject.downField("targetField").downArray.select)
```