Handle additionnal data (not only id and name)

If data provided by the query contains more than just an id and a name, we might want to collect them on form submition.

Example :

```{
    "id": 12,
    "name": "Alabama",
    "country_code": "US"
}```

With this commit, the option `returnAsJson`, if `true`, makes the form submit the full objects in a json string, instead of juste ids between separators. Example :

```
[{"id": 12, "name": "Alabama", "country_code": "US"},{"id": 16, "name": "Alberta", "country_code": "CA"}]
```
instead of
```
12,16
```

Optionnaly, new callback `dataFilter` allows data transformation on client side.