feat: add --outgoing-ignore-tls to keep the relay off STARTTLS (#377)

The relay connection only had `secure`, so with `secure: false` nodemailer
still upgraded to STARTTLS whenever the server advertised it, and there was
no way to talk cleartext to such a server. Pass nodemailer's `ignoreTLS`
through as `ignoreTLS` on the relay config, wired to
--outgoing-ignore-tls / MAILDEV_OUTGOING_IGNORE_TLS / `outgoingIgnoreTls`.

docs/api.md

@@ -68,6 +68,7 @@ const maildev = new MailDev({
   outgoingUser: 'user',
   outgoingPass: 'pass',
   outgoingSecure: true,
+  outgoingIgnoreTls: false, // Never upgrade the relay connection with STARTTLS
   autoRelay: true,         // Auto-forward emails
 
   // Logging