Revert "certificate: Remove Subject from CreateOptions"

This reverts commit ce649ecc66.

examples/cert.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"crypto/x509/pkix"
 	"fmt"
 	"log"
 	"os"
@@ -24,6 +25,9 @@ func main() {
 		log.Fatal(err)
 	}
 	options := certificate.CreateOptions{
+		Subject: pkix.Name{
+			CommonName: host,
+		},
 		DNSNames: []string{host},
 		Duration: duration,
 	}