Decouple demo site from the library core

[sgobotta]

Is your feature request related to a problem? Please describe.
We'd like to decouple the demo site from the library source code so that we can really test the core components in a closer user-like scenario. The goal is to use vue-admin as a dependency of the web demo site (e.g.: import { Admin, Resouce } from 'vue-admin-js' )

Describe the solution you'd like
Modularising the project with lerna is an option, though I'm not sure how can we achieve this core dependency usage in a demo site