Incorporate post-aggregation features

_From @thcrock on July 19, 2017 22:2_

We sometimes have the need to create features that come after aggregation but don't go through collate, or would be onerous to produce in collate. We should add support for this. I'm thinking that it could be treated like a new group table in the same shape as the collate tables that gets merged by the Builder in the same way.

This will probably involve renaming FeatureGenerator into something like AggregateFeatureGenerator, to avoid name confusion. It also might make sense to wrap all of the feature stuff (feature dictionary creation and grouping) in a larger component that handles all of it.

_Copied from original issue: dssg/architect#11_