Pipe and -conf flag together are an error (#315)

main.go

@@ -117,6 +117,19 @@ func mustLogFatal(args ...interface{}) {
 }
 
 func loadCaddyfile() (caddy.Input, error) {
+	// First try stdin pipe
+	cdyfile, err := caddy.CaddyfileFromPipe(os.Stdin)
+	if err != nil {
+		return nil, err
+	}
+	if cdyfile != nil {
+		// it is an error if -conf is also specified because, which to use?
+		if conf != "" {
+			return nil, errors.New("load: can't choose between stdin pipe and -conf flag")
+		}
+		return cdyfile, err
+	}
+
 	// -conf flag
 	if conf != "" {
 		contents, err := ioutil.ReadFile(conf)