Add support for Golang Weekly newsletters (#96)

* abstract different newsletter handler to a class

* create a generic newsletter handler class

* add support for golang-weekly newsletters

* add tests for golang weekly newsletters

* use addressparser lib to parse author from from address

packages/inbound-email-handler/src/substack-handler.ts

@@ -0,0 +1,13 @@
+import { NewsletterHandler } from './newsletter'
+
+export class SubstackHandler extends NewsletterHandler {
+  getNewsletterUrl(rawUrl: string, _html: string): string | undefined {
+    // raw SubStack newsletter url is like <https://hongbo130.substack.com/p/tldr>
+    // we need to get the real url
+    return rawUrl.slice(1, -1)
+  }
+
+  isNewsletter(rawUrl: string, _from: string): boolean {
+    return !!rawUrl
+  }
+}