Support new collate arithmetic functionality

Collate supports arithmetic on the computed aggregates to make new aggregates, as of https://github.com/dssg/collate/pull/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.

Start by reading the existing Triage feature configuration and the Collate arithmetic implementation referenced in pull request #63. Determine how overloaded aggregate operators should be represented in feature configuration, then define and validate the intended arithmetic semantics for Triage; the work is done when supported expressions are specified and covered by the project’s existing validation or test process.

Written by the indexing model from the issue text.