Add client certs and SASL EXTERNAL

examples/irccat.json

     "server": "irc.example.com:6697",
     "tls": true,
     "tls_skip_verify": false,
+    "tls_client_cert": "",
+    "tls_client_key": "",
     "nick": "irccat",
     "realname": "IRCCat",
     "server_pass": "",
     "identify_pass": "",
+    "sasl_external": false,
     "sasl_login": "",
     "sasl_pass": "",
     "channels": ["#channel"],

go.mod

 	github.com/spf13/pflag v1.0.5 // indirect
 	github.com/spf13/viper v1.5.0
 	github.com/stretchr/testify v1.3.0 // indirect
-	github.com/thoj/go-ircevent v0.0.0-20190807115034-8e7ce4b5a1eb
-	golang.org/x/sys v0.0.0-20191120155948-bd437916bb0e // indirect
+	github.com/thoj/go-ircevent v0.0.0-20210723090443-73e444401d64
+	golang.org/x/net v0.0.0-20210929193557-e81a3d93ecf6 // indirect
+	golang.org/x/text v0.3.7 // indirect
 	gopkg.in/go-playground/webhooks.v5 v5.13.0
 	gopkg.in/yaml.v2 v2.2.7 // indirect
 )

go.sum

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irc.go

 		irccon.SASLPassword = viper.GetString("irc.sasl_pass")
 		irccon.UseSASL = true
 	}
+	if viper.GetBool("irc.sasl_external") {
+		irccon.SASLMech = "EXTERNAL"
+		irccon.UseSASL = true
+	}
 
 	if viper.GetBool("irc.tls_skip_verify") {
 		irccon.TLSConfig = &tls.Config{InsecureSkipVerify: true}
 	}
+	if err := addClientCert(irccon); err != nil {
+		return err
+	}
 	irccon.Password = viper.GetString("irc.server_pass")
 
-	err := irccon.Connect(viper.GetString("irc.server"))
-	if err != nil {
+	if err := irccon.Connect(viper.GetString("irc.server")); err != nil {
 		return err
 	}
 
 		i.channels.Add(channel)
 	}
 }
+
+func addClientCert(irccon *irc.Connection) error {
+	certFile := viper.GetString("irc.tls_client_cert") // "" when unset
+	if certFile == "" {
+		return nil
+	}
+	keyFile := certFile
+	if k := viper.GetString("irc.tls_client_key"); k != "" {
+		keyFile = k
+	}
+	cert, err := tls.LoadX509KeyPair(certFile, keyFile)
+	if err != nil {
+		return err
+	}
+	existing := irccon.TLSConfig.Certificates
+	irccon.TLSConfig.Certificates = append(existing, cert)
+	return nil
+}