Add --cert-name option

[nichivo]

I have lots of somewhat related domains in SAN certificates but there isn't really a single "primary" domain that indicates what other domains are in the certificate. It would be nice to be able to have certificate filenames that indicate their purpose rather than just use the first domain from the SAN list.

# this patch adds --cert-name to be used when issuing certs only
acme.sh --issue --cert-name demo-sites ...
# other functions like renewal still use --domain
acme.sh --renew --domain demo-sites

I've been using this patch for a while, and looks like there's at least one other person that wants it too! Should fix https://github.com/acmesh-official/acme.sh/issues/4366