Add group chat functions, slight refactoring

src/structures/PrivateChat.js

@@ -0,0 +1,13 @@
+'use strict';
+
+const Chat = require('./Chat');
+
+/**
+ * Represents a Private Chat on WhatsApp
+ * @extends {Chat}
+ */
+class PrivateChat extends Chat {
+
+}
+
+module.exports = PrivateChat;