Missing X509v3 extensions field and/or missing subjectAltName are not reported

[briansmith]

According to briansmith/webpki#90, certlint doesn't indicate when the X509v3 extension field is missing, nor does it indicate when the subjectAltName is missing. Especially since web browsers and other software now routinely reject certificates without these fields, it would be good for certlint to point out when they are missing.