Added message duration in seconds (#1230)

Co-authored-by: Pedro S. Lopez <pedroslopez@me.com>

src/structures/Message.js

@@ -134,6 +134,12 @@ class Message extends Base {
          */
         this.hasQuotedMsg = data.quotedMsg ? true : false;
 
+        /**
+         * Indicates the duration of the message in seconds
+         * @type {string}
+         */
+        this.duration = data.duration ? data.duration : undefined;
+
         /**
          * Location information contained in the message, if the message is type "location"
          * @type {Location}