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.
Yeah, I like the name `lift` better than `toMonoidAggregator`.  I just chose a different name to avoid confusion.

You're also right that `preFold` is similar to `sumBefore`.  I think `preFold` is more general, but I think it's still worth keeping `sumBefore`, too.