Consider adding support for generating Diffie-Hellman parameters

Hey CF folks,

First of all, thanks for the excellent package. I've been moving over my [`hkjn/openvpn`](https://github.com/hkjn/openvpn) image and associated [`hkjn/pki`](https://github.com/hkjn/pki) helper to use keys and certs generated by your tools instead of ones from the (somewhat ironically named) [EasyRSA](https://github.com/OpenVPN/easy-rsa.git).

I couldn't find a way to use `cfssl` to generate Diffie-Hellman parameters, which is needed to set up OpenVPN.

I can of course use `openssl` for this:

```
openssl dhparam -out dh.pem 4096
```

But it would be nice if I didn't have to! That way, I could entirely get rid of the dependency on `openvpn`.

Let me know whether you think this would make sense.