Implements hook_field_formatter_settings_form_alter().
function field_test_field_formatter_settings_form_alter(&$element, &$form_state, $context) {
$element['field_test_formatter_settings_form_alter'] = array(
'#type' => 'textfield',
'#title' => t('Formatter settings form alter'),
'#default_value' => $context['formatter']
->getSetting('field_test_formatter_settings_form_alter'),
);
}