feat(iframe_api): Getter for the iframe

doc/api.md

 var numberOfParticipants = api.getNumberOfParticipants();
 ```
 
+You can get the iframe HTML element where Jitsi Meet is loaded with the following API function:
+```javascript
+var iframe = api.getIFrame();
+```
+
 You can remove the embedded Jitsi Meet Conference with the following API function:
 ```javascript
 api.dispose()

modules/API/external/external_api.js

         }
     }
 
+    /**
+     * Returns the iframe that loads Jitsi Meet.
+     *
+     * @returns {HTMLElement} The iframe.
+     */
+    getIFrame() {
+        return this.frame;
+    }
+
     /**
      * Returns the number of participants in the conference. The local
      * participant is included.