checking client certificate match
Hi,
In nodejs/express we can do the following:

[julie-ng source code](https://raw.githubusercontent.com/julie-ng/nodejs-certificate-auth/master/server/server.js)

```
app.get('/authenticate', (req, res) => {
	const cert = req.socket.getPeerCertificate();

	if (req.client.authorized) {
		res.send(`Hello ${cert.subject.CN}, your certificate was issued by ${cert.issuer.CN}!`);

	} else if (cert.subject) {
		res.status(403)
			 .send(`Sorry ${cert.subject.CN}, certificates from ${cert.issuer.CN} are not welcome here.`);

	} else {
		res.status(401)
		   .send(`Sorry, but you need to provide a client certificate to continue.`);
	}
});
```
what would be the equivalent in koa to verify client certificate?

