custom http code
Currently koa seems to only handle standard code.

When i use a code like 420 with `ctx.throw('custom message', 420)` , koa replace it by a 500.
When i use ...
```javascript
const err = new Error('custom message');
err.status = 420;
err.expose = true;
throw err;
```
... koa throw the error `invalid status code: 420`

I need to use custom code. Is there a way to add custom code to koa ?