Support multiple domain names

[lightmaster]

I'm in the process of changing my domain name, and it would be really nice to have a cert that covers both while I updated DOH/DOT devices.

I inputted for the domain name "dns.example.com,dns.differentexample.com,*.dns.differentexample.com" and while the current code works (albeit very clunkily, requesting for differentdomain.com multiple times) to get the cert on Let's Ecrypt's side, it has issues trying to save the file name as ./.lego/certificates/_.dns.example.com,dns.differentexample.com,*.dns.differentexample.com.key.

A simple fix ~~(although I do not know how to do this myself)~~ (See PR below) would be to split the domain name by commas and only save it as the first domain name.