feat/add js source mapping

alt + r for dev tool update
webpack/webpack#2478

src/index.js

@@ -2,11 +2,21 @@ import React from 'react';
 import {render} from 'react-dom';
 
 class Root extends React.Component {
+  constructor(props) {
+    super(props);
+    this.click = this.click.bind(this);
+  }
+
+  click() {
+    alert('click this');
+  }
+
   render() {
     return (
       <div>
         <h1> Hello world</h1>
         <input type="checkbox"/>
+        <button onClick={this.click}> try this</button>
       </div>
     );
   }

webpack.dev.js

@@ -1,4 +1,5 @@
 const base = require('./webpack.base.js'),
+  webpack = require('webpack'),
   dev = {
     module: {
       loaders: [{
@@ -10,7 +11,13 @@ const base = require('./webpack.base.js'),
           presets: [ 'es2015', 'react', 'react-hmre' ]
         }
       }]
-    }
+    },
+    plugins: [
+      new webpack.SourceMapDevToolPlugin({
+        filename: '[name].js.map',
+        exclude: ['vendor.js']
+      })
+    ]
   };
 
 module.exports = Object.assign(base, dev);