Add subtitle field to item fields list

Added 'subtitle' field to the parsed item fields in fields.js to support
RSS feeds that include subtitle information for items.

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

Author: openhands-agent

lib/fields.js

@@ -39,6 +39,7 @@ fields.item = [
   'pubDate',
   'author',
   'summary',
+  'subtitle',
   ['content:encoded', 'content:encoded', {includeSnippet: true}],
   'enclosure',
   'dc:creator',