gen.go: allow to customise the URL when generating

"go run gen.go" now takes the URL from the command line.
That URL is persisted in the go:generate line produced in certifi.go, so
"go generate" will reuse that URL forever.

This allows to easily fork the project to use a different URL (such as
the original Mozilla) if someone doesn't trust mkcert.org.

gen.go

@@ -8,12 +8,17 @@ import (
 	"log"
 	"net/http"
 	"os"
+	"strings"
 	"text/template"
 	"time"
 )
 
 func main() {
-	const url = "https://mkcert.org/generate/"
+	if len(os.Args) != 2 || !strings.HasPrefix(os.Args[1], "https://") {
+		log.Fatal("usage: go run gen.go <url>")
+	}
+	url := os.Args[1]
+
 	resp, err := http.Get(url)
 	if err != nil {
 		log.Fatal(err)
@@ -53,7 +58,7 @@ var tmpl = template.Must(template.New("").Parse(`// Code generated by go generat
 
 package gocertifi
 
-//go:generate go run gen.go
+//go:generate go run gen.go "{{ .URL }}"
 
 import "crypto/x509"