Mark fields as initially hidden in the JSON

[Elvis Nunez]

Would be great if you could mark a field as hidden by adding a "hidden" flag to the field's json.

{
  "id": "first_name",
  "hidden": true
}

References #317

[Matteo Crippa]

got it, plan to push tonight the solution

[Elvis Nunez]

Released on 3.0.2 => https://github.com/hyperoslo/Form/releases/tag/3.2.0

Thanks for your contribution @matteocrippa, you are awesome!