Add subtitle field to feed and item fields

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

lib/fields.js

@@ -8,6 +8,7 @@ fields.feed = [
   ['dc:title', 'title'],
   ['dc:type', 'type'],
   'title',
+  'subtitle',
   'description',
   'author',
   'pubDate',
@@ -35,6 +36,7 @@ fields.item = [
   ['dc:source', 'source'],
   ['dc:title', 'title'],
   'title',
+  'subtitle',
   'link',
   'pubDate',
   'author',