Add require_tls input to prevent STARTTLS fallback to plain text (#317)

Expose nodemailer's requireTLS option so that when secure is false
(e.g. STARTTLS on port 587), the connection aborts if the server
does not support/negotiate STARTTLS, instead of silently sending
credentials in clear text.

README.md

@@ -37,6 +37,12 @@ Some features:
     # Optional whether this connection use TLS (default is true if server_port is 465)
     secure: true
 
+    # Optional: when secure is false (e.g. STARTTLS on port 587), abort the
+    # connection if the server does not support/negotiate STARTTLS, instead
+    # of silently falling back to a plain text connection. Recommended
+    # whenever secure is false and you authenticate with username/password.
+    require_tls: true
+
     # Optional (recommended) mail server username:
     username: ${{secrets.MAIL_USERNAME}}