Allow multiple sorting for each column

[morlandi]

Use case: you have a list of addresses like:

street	number
aaa	3
aaa	10
aaa	10/A
aaa	2/B

Suppose that an hidden "normalized_number" has been added to support "natural sorting":

street	number	normalized_number
aaa	3	00000003
aaa	10	00000010
aaa	10/A	00000010/A
aaa	2/B	00000002/B

In this case, you wish to show (street + number) but sort on (street, normalized_number).

This might be solved with the optional "sort_field" column attribute, i.e.:

'sort_field': 'street,normalized_number'

then refactoring sort_queryset() accordingly