Default options form that provides the label widget that all fields should have.
Overrides FieldPluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
$form['machine_name'] = array(
'#title' => t('Output machine name'),
'#description' => t('Display field as machine name.'),
'#type' => 'checkbox',
'#default_value' => !empty($this->options['machine_name']),
);
}