Add support for publising to CDN

[DrSensor]

I happen to had a project that need to load component at runtime. Some way to do that is to use Async Component. To do that I need to build my component and save it in local disk or build it as UMD to be able to served in https://unpkg.com when publishing to npm. Luckily, poi support build component as UMD by

poi build --format umd --dist dist/umd

Reference:

• https://github.com/InCuca/vue-standalone-component/blob/master/template/package.json

[DrSensor]

Edit, there is official --library flag. Seems like poi has inconsistency issue.