fix #1238: accept multi-value input on sclice flags

flag.go

@@ -390,3 +390,7 @@ func flagFromEnvOrFile(envVars []string, filePath string) (val string, ok bool)
 	}
 	return "", false
 }
+
+func flagSplitMultiValues(val string) []string {
+	return strings.Split(val, ",")
+}