Add new transport option `state`
Koa is based on middleware. This means middleware can run after the Grant middleware to process the response data instead of a callback URL. I suggest to add a new `transport` option named `state`. This makes Grant assign the response data to `ctx.state.grant.response` instead of `ctx.body`.

Current usage:

```js
import qs from 'qs';

export default async (ctx, next) => {
  // Runs grant middleware;
  await next();

  const response = qs.parse(ctx.body);
}
```

New usage:

```js
export default async (ctx, next) => {
  // Runs grant middleware;
  await next();

  const { response } = ctx.state.grant;
}
```