Allow external activity tracking (closes snowplow#559)

Author: Dean Collins

src/js/tracker.js

@@ -1756,6 +1756,14 @@
 				configHeartBeatTimer = heartBeatDelay * 1000;
 			},
 
+			/**
+			 * Triggers the activityHandler manually to allow external user defined
+			 * activity. i.e. While watching a video
+			 */
+			updatePageActivity: function () {
+				activityHandler();
+			},
+
 			/**
 			 * Enables automatic form tracking.
 			 * An event will be fired when a form field is changed or a form submitted.