Add JoinMessage::GetKey() & SetKey()

This will be useful for implementing the upcoming OnUserJoinMessage()
module hook, that is allowed to modify the key.

include/znc/Message.h

@@ -107,6 +107,8 @@ public:
 
 class CJoinMessage : public CTargetMessage {
 public:
+	CString GetKey() const { return GetParam(1); }
+	void SetKey(const CString& sKey) { SetParam(1, sKey); }
 };
 
 class CNickMessage : public CMessage {