hooks.invoice_payment: New hook.

In the long run, this should be some kind of configurable hook to run
arbitrary regexps on arbitrary entry fields, and then on a match, add
another field with a format string using the match data.  But this is
reasonably generic for now.

import2ledger/hooks/invoice_payment.py

@@ -0,0 +1,23 @@
+class InvoicePaymentHook:
+    def __init__(self, config):
+        pass
+
+    def run(self, entry_data):
+        try:
+            words = iter(entry_data['description'].lower().split())
+        except KeyError:
+            return
+        want_words = iter(['payment', 'for', 'invoice'])
+        want_next = next(want_words)
+        for have_word in words:
+            if have_word == want_next:
+                try:
+                    want_next = next(want_words)
+                except StopIteration:
+                    try:
+                        last_word = next(words)
+                    except StopIteration:
+                        pass
+                    else:
+                        entry_data['invoice_id'] = last_word.lstrip('#')
+                    break