webpack升級到4.0.1，執行webpack命令報錯：
>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 按錯誤提示，webpack單獨分離出了wepack-cli，需要我們安裝wepack-cli。
全局安裝：
npm i -g webpack-cli 全局安裝webpack-cli后，使用webpack -v成功。
在項目本地安裝webpack，webpack-cli：
npm install webpack webpack-cli -D