Allow multi-category transactions

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

pfbudget/transform/categorizer.py

@@ -6,8 +6,8 @@ from pfbudget.db.model import (
     CategorySelector,
     Transaction,
     TransactionCategory,
+    TransactionTag,
 )
-from .exceptions import TransactionCategorizedError
 from .transform import Transformer
 
 
@@ -24,12 +24,15 @@ class Categorizer(Transformer):
     def transform_inplace(self, transactions: Sequence[Transaction]) -> None:
         for rule in self.rules:
             for transaction in transactions:
-                if transaction.category:
-                    raise TransactionCategorizedError(transaction)
-
                 if not rule.matches(transaction):
                     continue
 
+                if not transaction.category:
                     transaction.category = TransactionCategory(
                         rule.name, CategorySelector.rules
                     )
+                else:
+                    if not transaction.tags:
+                        transaction.tags = {TransactionTag(rule.name)}
+                    else:
+                        transaction.tags.add(TransactionTag(rule.name))

pfbudget/transform/exceptions.py

@@ -1,6 +1,2 @@
 class MoreThanOneMatchError(Exception):
     pass
-
-
-class TransactionCategorizedError(Exception):
-    pass