Marks this element as a from group, which (usually) contains a label, input and error message.
You are not required to use form groups to submit forms through Unpoly.
However, structuring your form into groups will help Unpoly to make smaller changes to the DOM when
working with complex form. For instance, when validating a field,
Unpoly will re-render the closest
form group around that field.
By default Unpoly will also consider a <fieldset> or <label> around a field to be a form group.
You can configure this in up.form.config.groupSelectors.
