fix: get React Hooks working!

Add the `setConfig` fix from gatsbyjs/gatsby#9489 (comment) to get Hooks working in develop mode using hot module reloading. This may cause issues where the data gets out of sync, but it’s the best we can hope for right now.

src/pages/index.js

@@ -1,4 +1,7 @@
 import React, { useState } from 'react';
+import { setConfig } from 'react-hot-loader';
+
+setConfig({ pureSFC: true });
 
 export default () => {
   const [count, setCount] = useState(0);