New feature: SMTP connection may use client certificate.

New config keys in [mailer] section:
 - CERT_FILE: path to a certificate file.
 - KEY_FILE: path to a key file.

modules/mailer/mailer.go

@@ -72,9 +72,15 @@ func sendMail(settings *setting.Mailer, recipients []string, msgContent []byte)
 		return err
 	}
 
+	cert, err := tls.LoadX509KeyPair(settings.CertFile, settings.KeyFile)
+	if err != nil {
+		return err
+	}
+
 	tlsconfig := &tls.Config{
 		InsecureSkipVerify: settings.SkipVerify,
 		ServerName:         host,
+		Certificates:       []tls.Certificate{cert},
 	}
 
 	conn, err := net.Dial("tcp", net.JoinHostPort(host, port))