How to use this middleware with koa-bodyparser
Hello and thank you all for the work done on this project.

I'm trying to integrate this middleware into a project that already uses [koa-bodyparser](https://github.com/koajs/bodyparser) (required middleware to [koa-passport](https://www.npmjs.com/package/koa-passport)).

Unfortunately when trying to reach a route when this middleware is declared the road never answers. I guess there is a conflict with `co-body` somewhere. 

Do we have a workaround for using this middleware with `koa-bodyparser` or am i going wrong ?

``` javascript
const koa = require('koa');
const bodyParser = require('koa-bodyparser');
const router = require('koa-joi-router');
const Joi = router.Joi;

const app = koa();
const helloRouter = router();

app.use(bodyParser()); // <-- comment and the call will work

helloRouter.route({
  method: 'post',
  path: '/hello',
  validate: {
    body: {
      name: Joi.string().required()
    },
    type: 'json'
  },
  handler: function* hello() {
    const { name } = this.request.body;
    this.body = `Hello ${name}`;
  }
});

app.use(helloRouter.middleware());

app.listen(3000);
```

The call : `curl -X POST -H "Content-Type: application/json" -d '{ "name": "John" }' "http://localhost:3000/hello"`
