Enable hot module replacement(HMR) (#1100)

* Use ES6 in static/main.js

+ Lint static/main.js

* Enable hot module replacement(HMR) in webpack-dev-server

* Disable no-console in static/main.js

* Use seperate entry point/bundle for the test page(/static/main.js)

webpack.dev.js

@@ -3,10 +3,14 @@ const { targets, createConfig } = require('./webpack.common');
 const path = require('path');
 const PORT = 7936;
 
-// only the `devServer` options for the first configuration will be taken
-// into account and used for all the configurations in the array.
 //                                                dev   minify
 const katexConfig = createConfig(targets.shift(), true, false);
+
+// add the entry point for test page
+katexConfig.entry.main = './static/main.js';
+
+// only the `devServer` options for the first configuration will be taken
+// into account and used for all the configurations in the array.
 katexConfig.devServer = {
     contentBase: [path.join(__dirname, 'static'), __dirname],
     // Allow server to be accessed from anywhere, which is useful for