Allow exporting change list verbatim

[Trey Hunner]

Example which does not work because values in "headers" are expected to be model fields:

class SomeModelAdmin(TablibAdmin):
    list_display = ['id', 'name']
    formats = ['xls', 'csv']
    headers = list_display

    def name(self, obj):
        return "{0} {1}".format(obj.first_name, obj.last_name)

The fields could be rendered similarly to the way the change list fields are rendered.