Drupal views: Make format filter configuration in civicrm_handler_field_markup

At present, the markup handler provided by civicrm (civicrm_handler_field_markup) uses the default Drupal format filter. In general, this is 'plain text' and in general it's not a good idea to modify this either. This has the unfortunate consequence that the markup filter can't actually handle markup (ie. html mark is rendered as html entities).

The attached patch allows the admin to specify a particular text format filter for a civicrm field using the markup handler.