docs: update reactjs demo for consistant ES6 style

demo/reactjs/src/engine.js

@@ -0,0 +1,12 @@
+import path from 'path';
+import { Liquid } from 'liquidjs';
+
+export const engine = new Liquid({
+    root: path.resolve(__dirname, 'views/'),  // dirs to lookup layouts/includes
+    extname: '.liquid'     // the extname used for layouts/includes, defaults 
+});
+
+engine.registerFilter('image', d => {
+  let img = `<img src="${d}" class="App-logo" alt="logo"></img>`;  
+  return img 
+})