on-modify.timewarrior: Use last annotation as additional tag, if present

This allows the following workflow to be effective and tracked in a useful manner:
```
task 123 start First draft
task 123 stop
...
task 123 start Second iteration
task 123 stop
...
task 123 start Bugfixing
task 123 stop
...
```

This will be tracked as:
```
01:00 - 02:00 <TASK 123 TAGS> "First draft"
03:00 - 04:00 <TASK 123 TAGS> "Second iteration"
05:00 - 06:00 <TASK 123 TAGS> "Bugfixing"
```

on_modify.py

@@ -59,6 +59,9 @@ def extract_tags_from(json_obj):
         else:
             tags.extend(json_obj['tags'])
 
+    if 'annotations' in json_obj:
+        tags.append(json_obj['annotations'][-1]['description'])
+
     return tags