Add subtitle field to feed fields

- Add 'subtitle' to fields.feed array in lib/fields.js
- Add subtitle property to TypeScript Output interface in index.d.ts

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

Author: openhands-agent

index.d.ts

@@ -59,6 +59,7 @@ declare namespace Parser {
     items: (U & Item)[];
     feedUrl?: string;
     description?: string;
+    subtitle?: string;
     itunes?: {
       [key: string]: any;
       image?: string;

lib/fields.js

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