[Koa] custom properties

example:
```
const config = require('./config')
const app = new Koa();
app.context.config = config;

app.use((ctx, next) => {
    // use `config` object
    const config = ctx.config
})
```

types/koa/index.d.ts

@@ -694,6 +694,7 @@ declare namespace Application {
          * To bypass Koa's built-in response handling, you may explicitly set `ctx.respond = false;`
          */
         respond?: boolean;
+        [key: string]: any;
     }
 }