Allow flat results on aggregates

[agateblue]

Right now, aggregate always return a dictionary:

manager.filter(gender='female').aggregate(lifter.Avg('age'), lifter.Min('age'))
# >>> {'age__sum': 289, 'age__min': 15}

As suggested on reddit, it would be nice to return a flat list:

manager.filter(gender='female').aggregate(lifter.Avg('age'), lifter.Min('age'), flat=True)
# >>> [289, 15]

This would enable unpacking on aggregates:

avg_age, min_age = manager.filter(gender='female')\
                          .aggregate(lifter.Avg('age'), lifter.Min('age'), flat=True)