Added support for updating counter columns

Right - my point is that saying `value=20` does not do the obvious thing.  It does not set the value to 20.

I'd rather use something like `value__incr=20`, similar to the blind operations on sets https://cqlengine.readthedocs.org/en/latest/topics/queryset.html?highlight=blind#cqlengine.query.QuerySet.update