nagyandras

Support for adding (subtracting) a custom value would be nice. `topn_add(jsonb, text, integer)`

Support for negative counters would be nice. ``` select topn_union('{"asd":-1}'::jsonb, '{"asd":2}'::jsonb) select topn_add('{"asd":-1}'::jsonb, 'asd') topn_union_agg(...) ``` They overflow now.