Allow expanding certificates

[dchenk]

Certbot has an --expand option that lets you add domains to a certificate: https://certbot.eff.org/docs/using.html#re-creating-and-updating-existing-certificates

Is there any chance we could implement such a feature for lego? The advantage with simply expanding a certificate's list of domains is that you don't then get emails notifying you that your (old) certificate is about to expire when you're not using it anymore after creating a new cert with another list of domains. (Or am I wrong, does the old cert expire as usual?)

[xenolf]

Hello!

There is no such thing as expanding a certificate really. Once a certificate is issued it will remain the same forever. What the --expand option probably does is that it reads the domains from the certificate and generates a new one based on the domains it found plus any additional ones you provided.

I'm not opposed to adding such functionality to the CLI as I can see the benefit of it. Happy to accept a PR for it!