webpack watch模式产生*.hot-update.json文件

webpack --watch会产生*.hot-update.json文件，解决方法如下：

output: {
    path: path.join(root, "dist"),
    filename: "bundle.js",
    hotUpdateChunkFilename: 'hot/hot-update.js',
    hotUpdateMainFilename: 'hot/hot-update.json'
}

出处：https://github.com/gaearon/react-hot-loader/issues/456