Unable to extend the app's context in Koa@2

Trying to extend the context of the application since migrating to koa@2 throws this error:

```
TypeError: Cannot set property 'property_name' of #<Object> which has only a getter
```

A small example code

``` javascript
import Koa from 'koa';

const app = new Koa();
app.context.host = process.env.HOST || '0.0.0.0';
app.context.port = process.env.PORT || 8080;
```
