* irssi: add ssl_cert option for servers
I was following these instructions
https://www.oftc.net/NickServ/CertFP/
and found that the `/server add -ssl_cert` option was needed.
This patch therefore adds an optional
`programs.irssi.networks.<name>.server.ssl.certificateFile` path.
Perhaps this could also be done with a `settings` attribute, but that
would probably require most of this module to be reworked.
* irsii: Add example-settings test case
