Support email addresses in subjectAlternativeName

[njsmith]

I think we could just use a silly rule like "if a 'hostname' has an @ in it, that's an email address".

urllib3 is an example of a project that uses test certs with email addresses in them.

(This might be the only difference between client certs and server certs in practice? Maybe issue_server_cert is a misnomer and it should be renamed to issue_cert?)