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.
Cool, I now see the method implementations that are unsafe in the general Aggregator trait. As I was saying off thread to @avibryant , I definitely see a lot of appeal in making this library code as flexible as possible. I'll update `lift` and the tests accordingly tonight.