Allow localpart as username

If username is not a JID assume it is the local part and append an @
and the server domain.

go-sendxmpp.go

@@ -136,6 +136,19 @@ func parseConfig(configPath string) (configuration, error) {
 	}
 	file.Close()
 
+	// Check if the username is a valid JID
+	output.username, err = MarshalJID(output.username)
+	if err != nil {
+		// Check whether only the local part was used by appending an @ and the
+		// server part.
+		output.username = output.username + "@" + output.jserver
+		// Check if the username is a valid JID now
+		output.username, err = MarshalJID(output.username)
+		if err != nil {
+			return output, errors.New("invalid username/JID: " + output.username)
+		}
+	}
+
 	return output, err
 }