Feature/aggregate multi params

[szykin]

Aggregate now accepts multiple positional and keyword arguments, just like in Django.
Second commit is a quick fix so distinction of positional arguments will happen before loop.
If you think some of the tests I added here are overkill, feel free to remove them.

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #24 into master will not change coverage

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[stphivos]

Thanks! Yeah all the tests seem useful here and serve a different purpose, except maybe the first (test_query_aggregate_with_none_only_field_values_performs_correct_aggregation)
seems like a subset of the third (test_query_aggregate_multiple_params_with_none_only_field_values_aggregation_with_none)? But even so we can clean it up in the future if we determine it's not needed!