Check certificates for nearing expiry.
$ go run check-certs.go -days 100 igor.io gif.industries expired.badssl.com
error: expired.badssl.com: tls dial: x509: certificate has expired or is not yet valid
error: gif.industries: cert[0] gif.industries expires at 2016-12-10 11:00:00 +0000 UTC
error: igor.io: cert[0] igor.io expires at 2016-12-10 11:00:00 +0000 UTC
exit status 1