Provide a form to edit options for this plugin.
Overrides DefaultSummary::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
$form['inline'] = array(
'#type' => 'checkbox',
'#default_value' => !empty($this->options['inline']),
'#title' => t('Display items inline'),
);
$form['separator'] = array(
'#type' => 'textfield',
'#title' => t('Separator'),
'#default_value' => $this->options['separator'],
);
}