Implement joinChannel command

joinChannel moves the current user into the given channel identified by
channel_id.

include/mumlib.hpp

@@ -41,6 +41,8 @@ namespace mumlib {
 
         void sendTextMessage(std::string message);
 
+        void joinChannel(int channel_id);
+
     private:
         _Mumlib_Private *impl;
     };

src/mumlib.cpp

@@ -371,4 +371,10 @@ namespace mumlib {
         textMessage.set_message(message);
         impl->transport.sendControlMessage(MessageType::TEXTMESSAGE, textMessage);
     }
+
+    void Mumlib::joinChannel(int channel_id) {
+        MumbleProto::UserState userState;
+        userState.set_channel_id(channel_id);
+        impl->transport.sendControlMessage(MessageType::USERSTATE, userState);
+    }
 }