[Feature] Preselect only some fields for export

[gabrieletassoni]

Is your feature request related to a problem? Please describe.
I can't figure out how to have all but a few fields unchecked in the export page. When I do an Export Selected Items (for example) I get this:

I would be great if I can configure a model just to have pre-selected only some of the fields, like this:

Describe proposed solution(s)
A way to define all the checkboxes as unchecked and configure only some fields as checked, something like:

rails_admin do
  export do
    # Unchecks all checkboxes
    configure_all_checked false

    configure :name do
      checked true
    end
  end
end