tls_expiry: enable sni by default and use host for that?

[cpina]

the tls_expiry monitor: SNI seems quite common nowadays. Instead of having:

[lets-encrypt-certificat-pina-cat-28days]
type=tls_expiry
host=pina.cat
min_days=28
sni=pina.cat

Could be:

[lets-encrypt-certificat-pina-cat-28days]
type=tls_expiry
host=pina.cat
min_days=28

I think that SNI is used by default everywhere (or is part of the handshake when the browser connects there?).

I know that is hard to change default options without breaking existing monitors...