More granular blind updates to collections

Implements #156

Allows blind update to append/prepend to List, add/remove from Set, and merge into a Map.

E.g.
    SomeModel.objects(myid=123).update(myset__add={"foo", "bar"})
    SomeModel.objects(myid=123).update(mylist__append=["foo", "bar"])
    SomeModel.objects(myid=123).update(mymap__merge={"foo": "bar"})

I'm planning to add documentation for this (a "Storing data" section in the "Making Queries" docs https://cqlengine.readthedocs.org/en/latest/topics/queryset.html) but wanted to open the PR first to see if it looks ok.