Fixed some errors reported in google lighthouse audit tool.

config/webpack.config.js

@@ -3,6 +3,7 @@ var webpack = require('webpack');
 var path = require('path');
 var loaders = require('./webpack.loaders');
 var HtmlWebpackPlugin = require('html-webpack-plugin');
+var CopyWebpackPlugin = require('copy-webpack-plugin');
 
 const HOST = process.env.HOST || "127.0.0.1";
 const PORT = process.env.PORT || "8888";
@@ -59,6 +60,12 @@ module.exports = {
     new HtmlWebpackPlugin({
       title: 'Maputnik',
       template: './src/template.html'
-    })
+    }),
+    new CopyWebpackPlugin([
+      {
+        from: './src/manifest.json',
+        to: 'manifest.json'
+      }
+    ])
   ]
 };