Provide the default form for setting options.
Overrides PluginBase::buildOptionsForm
public function buildOptionsForm(&$form, &$form_state) {
parent::buildOptionsForm($form, $form_state);
$form['offset'] = array(
'#type' => 'textfield',
'#title' => t('Offset'),
'#description' => t('The number of items to skip. For example, if this field is 3, the first 3 items will be skipped and not displayed.'),
'#default_value' => $this->options['offset'],
);
}