feat: add_custom_postings to banking.py

From request here:
redstreet/reds-ramblings-comments#21 (comment)

beancount_reds_importers/libtransactionbuilder/banking.py

@@ -154,6 +154,7 @@ def extract(self, file, existing_entries=None):
             if target_acct:
                 data.create_simple_posting(entry, target_acct, None, None)
 
+            self.add_custom_postings(entry, ot)
             new_entries.append(entry)
 
         new_entries += self.extract_balance(file, counter)

beancount_reds_importers/libtransactionbuilder/investments.py

@@ -544,9 +544,6 @@ def add_fee_postings(self, entry, ot):
             if getattr(ot, "commission", 0) != 0:
                 data.create_simple_posting(entry, config["fees"], ot.commission, self.currency)
 
-    def add_custom_postings(self, entry, ot):
-        pass
-
     def extract_custom_entries(self, file, counter):
         """For custom importers to override"""
         return []

beancount_reds_importers/libtransactionbuilder/transactionbuilder.py

@@ -42,6 +42,10 @@ def set_config_variables(self, substs):
             filing_account = self.config["main_account"].format(**kwargs)
             self.config["filing_account"] = self.remove_empty_subaccounts(filing_account)
 
+    def add_custom_postings(self, entry, ot):
+        """This method is for importers to override. Add arbitrary posting to each entry."""
+        pass
+
     def build_metadata(self, file, metatype=None, data={}):
         """This method is for importers to override. The overridden method can
         look at the metatype ('transaction', 'balance', 'account', 'commodity', etc.)