through the use of tags. Instead of failing when categorizing a
transaction which already has a category, add the new category as a tag
for that transaction.
Issue #2
and allow for multiple null rules on the Nullifier.
Also create an empty list on the Nullifier constructor to simplify the
logic afterwards, in the case no "null" rule exist.
The `CategorySelector` was possibly added to be incremented with other
attributes. However, since none other that the selector enum is used at
the moment, it is only adding unnecessary cluter.
The category selector value is moved to the parent
`TransactionCategory`.
