Add subtitle field to feed and item field lists

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

Author: openhands-agent

lib/fields.js

@@ -9,6 +9,7 @@ fields.feed = [
   ['dc:type', 'type'],
   'title',
   'description',
+  'subtitle',
   'author',
   'pubDate',
   'webMaster',
@@ -35,6 +36,7 @@ fields.item = [
   ['dc:source', 'source'],
   ['dc:title', 'title'],
   'title',
+  'subtitle',
   'link',
   'pubDate',
   'author',