How to define own ctx.state types
I used typescript to devloper koa app,I want to define my own type for ctx.state in global of app,but i don't know how to do.
![QQ截图20190512150010](https://user-images.githubusercontent.com/7766085/57578885-b0332800-74c6-11e9-9734-91b8c33618e2.jpg)
