Allow custom properties to export

[krsoninikhil]

This is the simplest package I found for exporting data from admin.
Problem that I faced was that datetime fields get exported as UTC timezone, even if timezone is enabled in django settings. I tried adding a custom property in model but the way fields are taken in this package 0 only takes database fields.

queryset = queryset.values_list(*field_names)

Would you be open to have a PR on adding custom properties in exported data?

[vrocha]

Yes, either that or making the datatime fields export using the enabled timezone setting (my preference).