babel 将ES6 转化为 ES5 的命令: http://www.ruanyifeng.com/blog/2016/01/babel.html
$ babel example.js
$ babel example.js --out-file compiled.js
$ babel example.js -o compiled.js
$ babel src --out-dir lib
$ babel src -d lib
$ babel src -d lib -s
$ npm run startlib
0 - turn the rule off
1 - turn the rule on as a warning (doesn't affect exit code)
2 - turn the rule on as an error (exit code will be 1)