Provide a form to edit options for this plugin.
Overrides StylePluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
$form['description'] = array(
'#type' => 'textfield',
'#title' => t('RSS description'),
'#default_value' => $this->options['description'],
'#description' => t('This will appear in the RSS feed itself.'),
'#maxlength' => 1024,
);
}