es6 export-from用法
语法：
export * from "foo"; export {foo} from "foo"; export {foo, bar} from "foo"; export {foo as bar} from "foo"; export {foo as default} from "foo"; export {foo as default, bar} from "foo"; export {default} from "foo"; export {default as foo} from "foo";
注意：export-from 用于聚集模块。但不能在直接使用。例如在index.js里export { a } from 'b.js', a在index.js里无法使用
参考：http://blog.csdn.net/DeepLies/article/details/52916221?locationNum=13&fps=1