Allow dependent applications to programmatically set the current inde…

…x (#320)

As requested in prabhuignoto/react-chrono#300
(and also because I needed it 😄), this implements a simple `useEffect`
that updates the `activeTimelineIndex` accordingly.

src/components/index.tsx

@@ -98,6 +98,10 @@ const Chrono: React.FunctionComponent<Partial<TimelineProps>> = (
     }
   }, []);
 
+  useEffect(() => {
+    handleTimelineUpdate(activeItemIndex);
+  }, [activeItemIndex]);
+
   const restartSlideShow = useCallback(() => {
     setSlideshowActive(true);
     handleTimelineUpdate(0);