Provide a form to edit options for this plugin.
public function buildOptionsForm(&$form, &$form_state) {
// Some form elements belong in a fieldset for presentation, but can't
// be moved into one because of the form_state['values'] hierarchy. Those
// elements can add a #fieldset => 'fieldset_name' property, and they'll
// be moved to their fieldset during pre_render.
$form['#pre_render'][] = 'views_ui_pre_render_add_fieldset_markup';
}