Adding Mailroom madness for review in branch session12

This will cover two assignments - both the "create non running code" and the actual mail room madness code
Nice use of `dict.setdefault()`. However, there's an issue here that causes your code to fail with a TypeError if the following steps are taken:

1. `run mailroom.py`
1. `t`
1. `any string that's not in the dictionary`
1. `any sequence of digits`
1. `r`

This can either be corrected in one of the above two lines, or in the definition of `promt_donation_amount()`.