出现的问题：
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 i -D webpack-cli
-> When using yarn: yarn add -D webpack-cli
module.js:491
throw err;
^

Error: Cannot find module 'webpack-cli/bin/config-yargs'
at Function.Module._resolveFilename (module.js:489:15)
at Function.Module._load (module.js:439:25)
at Module.require (module.js:517:17)
at require (internal/module.js:11:18)
at Object.<anonymous> (E:\webpack_vue\node_modules\webpack-dev-server\bin\webpack-dev-server.js:87:1)
at Module._compile (module.js:573:30)
at Object.Module._extensions..js (module.js:584:10)
at Module.load (module.js:507:32)
at tryModuleLoad (module.js:470:12)
at Function.Module._load (module.js:462:3)

原因：
需要安装webpack-cli

解决方案：
$ npm install webpack-cli --save-dev