Example email forwarder with ARC support

1. Generate certificate/key files for STARTTLS and ARC signing
2. Edit configuration file, set port nr (should be 25) and key locations. Also configure virtual alias map
3. Install dependencies with npm install
4. Run the app sudo node server.js (sudo is required for port 25)

See an example message forwarded via this forwarding service running at forward.srv.dev here. Check out the ARC-* headers in that message.