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.
Let me try to sum up, so to speak. Sticking with current names as they exist, it sounds like the only change we're talking about is, for the case where you want to go from `Aggregator[A,B,C]` to `Aggregator[Traversable[A], Option[B], Option[C]`, you would now call `agg.toMonoidAggregator.sumBefore`, whereas in the original PR you would call `agg.lift`.

I'm happy to make that change if everyone likes it, but I'm personally not entirely convinced of the value of a `MonoidAggregator[A, Option[B], Option[C]` - I'm not sure I would ever use it without also calling `sumBefore` on it.