Exportable custom fields in display suite view mode

When displaying a CiviCRM custom field in a Display Suite view mode, in display or form,
and trying to export via features,
it can export the fields only in another site where the custom fields have the same ids

if you have two sites that are creating CiviCRM custom fields, they can't export view modes to one another.

I'll post a patch to handle the custom fields by names instead of ids