Split out TransportConfiguration

This allows using the same transport configuration from both clients and components.

config.go

@@ -1,12 +1,15 @@
 package xmpp
 
 import (
-	"crypto/tls"
 	"io"
 	"os"
 )
 
 type Config struct {
+	// TransportConfiguration must not be modified after having been passed to NewClient. Any
+	// changes made after connecting are ignored.
+	TransportConfiguration
+
 	Address        string
 	Jid            string
 	parsedJid      *Jid // For easier manipulation
@@ -14,9 +17,6 @@ type Config struct {
 	StreamLogger   *os.File // Used for debugging
 	Lang           string   // TODO: should default to 'en'
 	ConnectTimeout int      // Client timeout in seconds. Default to 15
-	// tls.Config must not be modified after having been passed to NewClient. Any
-	// changes made after connecting are ignored.
-	TLSConfig *tls.Config
 	// Insecure can be set to true to allow to open a session without TLS. If TLS
 	// is supported on the server, we will still try to use it.
 	Insecure      bool