webpack升级到4.0.1，执行webpack命令报错：
>webpack -v
The CLI moved into a separate package: webpack-cli.
Please install 'webpack-cli' in addition to webpack itself to use the CLI.
-> When using npm: npm install webpack-cli -D
-> When using yarn: yarn add webpack-cli -D
按错误提示，webpack单独分离出了wepack-cli，需要我们安装wepack-cli。
全局安装：
npm i -g webpack-cli
全局安装webpack-cli后，使用webpack -v成功。
在项目本地安装webpack，webpack-cli：
npm install webpack webpack-cli -D