Make app entry point Universal

- Render to DOM in a browser
 - Export a render function for the server

src/index.js

@@ -1,9 +1,16 @@
 import React from 'react';
 import ReactDOM from 'react-dom';
+import { renderToString } from 'react-dom/server';
 import App from './App';
 import './index.css';
 
-ReactDOM.render(
-  <App />,
-  document.getElementById('root')
-);
+if (typeof window !== 'undefined') {
+  ReactDOM.render(
+    <App />,
+    document.getElementById('root')
+  );
+}
+
+export default function render() {
+  return renderToString(<App />);
+}