Nick Becker's Mail Room Assignment

Nick Becker's mailroom.py assignment for 12/8
This function is called `new_donor()` but it handles updating donations for an existing donor. Consider renaming the function to make it's purpose and capabilities more obvious.

`Donors[name]` is a list. Instead of replacing that list with a new list (which is what `donations + Donors[name]` produces) consider modifying the existing list.