added default role - if defined replaces wholelist when role=None is passed to export_loop

This changeset adds a new feature for the default behavior for exporting data.  

Currently the default behavior if `role=None` is passed into the `export_loop` is to use `wholelist()`.  

This changeset will check for the presence of a `default` role in `Options.roles`.  If found, `gottago` is set to the value of `default` rather than `wholelist()` if `role=None`.