Lift methods for Aggregator and MonoidAggregator

This can be useful when you have a common call site that expects to use
the standard Aggregator API (prepare, present) but in some cases you
wish to aggregate values that come as Option or collection types.
Also, I do like the suggestion of having a separate method to convert an `Aggregator[A, B, C]` to `MonoidAggregator[A, Option[B], Option[C]]`