Rewrite to use CASL?

Hi there,

I'm just wondering whether you would like to use [casl](https://github.com/stalniy/casl) in your project.

This library implements cancan for JavaScript in isomorphic way + a bit more. This will allow to expand functionality of `egg-cancan`.

There are a bunch of articles about CASL but these ones may be the most interesting for you:
- https://medium.com/@sergiy.stotskiy/authorization-with-casl-in-express-app-d94eb2e2b73b
- https://blog.feathersjs.com/authorization-with-casl-in-feathersjs-app-fd6e24eefbff

I'm the author of this library, so feel free to ask questions if any.

Looking forward for productive collaboration!