Support new collate arithmetic functionality

[jesteria]

Collate supports arithmetic on the computed aggregates to make new aggregates, as of dssg/collate#63. Triage should support this too, especially since it comes up as a wanted feature every once in a while.

It's not just a new kwarg, it's implemented in collate by overloading the operators on the aggregators, so it's not entirely clear what the semantics in the feature config will be.

This issue originally referred to both arithmetic and subquery functionality, but #250 refers to the subquery functionality, so I repurposed this.