add javascript webhook implementation

.gitignore

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+*/**/node_modules

javascript/index.js

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+'use strict';
+const express = require('express');
+const bodyParser = require('body-parser');
+
+// The rest of the code implements the routes for our Express server.
+let app = express();
+
+app.use(bodyParser.json());
+app.use(bodyParser.urlencoded({
+  extended: true
+}));
+
+// Webhook validation
+app.get('/webhook', function(req, res) {
+    // replace <VERIFY_TOKEN> here with the verify token you specified on messenger dashboard
+    if (req.query['hub.mode'] === 'subscribe' &&
+        req.query['hub.verify_token'] === '<VERIFY_TOKEN>') {
+        res.status(200).send(req.query['hub.challenge']);
+    } else {
+        res.send('Error, wrong validation token');  
+    }
+});
+
+// Set Express to listen out for HTTP requests
+var server = app.listen(process.env.PORT || 3000, function () {
+  console.log("Listening on port %s", server.address().port);
+});