Returns the filtered values of a form.

They are basically:
- for named options
- one entry per input/checked/selected option (even if they have the same name)

@api
@returns {Array.<{name: String, label: String, value: String|Boolean|Number}>}
function getFormData(){
    var form = this.form;
    var selector = ['input', 'select', 'textarea'].join(',');

    return [].slice.call(form.querySelectorAll(selector))
      .filter(Mailto.formDataFilter)
      .map(Mailto.formDataMapper(form));
  }