Better support for wildcard domains

[JonasAlfredsson]

With the latest pull request we added support for DNS challenges, which means that we could now support wildcard domains. Creating a server config which looks like this:

server {
    listen              443 ssl;
    server_name         yourdomain.org *.yourdomain.org;
    ssl_certificate_key /etc/letsencrypt/live/test-name/privkey.pem;
    ...
}

actually produces a functioning wildcard certificate, but this is not really versatile. A possible solution to this is to make something like this possible:

server {
    listen              443 ssl;
    server_name         sub-1.yourdomain.org;
    ssl_certificate_key /etc/letsencrypt/live/yourdomain.org/privkey.pem; #wildcard
    ...
}

server {
    listen              443 ssl;
    server_name         sub-2.yourdomain.org;
    ssl_certificate_key /etc/letsencrypt/live/yourdomain.org/privkey.pem; #wildcard
    ...
}

where we pick up the trailing comment and make the certificate request for this certificate only be -d yourdomain.org -d *.yourdomain.org.

We probably need some more logic here to identify and extract when this should happen, and we most likely must demand that the cert_name = yourdomain.org, so we can skip the tedious thing of trying to guess the "base" domain from the server_name.

[XaF]

I'd recommend using the same approach we do for other matches at the moment, with a .wildcard or -wildcard somewhere in the cert name. As well as a env variable.
The only part where this gets slightly more involved is the "reduction", but that's actually not that complicated to do to handle.

[XaF]

Just created #94 that should solve this :)

[JonasAlfredsson]

This should now be included in the v3.0.0 release.