Feature: (issue_1451) customized slice flag separator (#1546)

* feat: customized slice flag separator

* feat: modify go doc

* feat: update unit test

flag.go

@@ -15,6 +15,8 @@ import (
 
 const defaultPlaceholder = "value"
 
+var defaultSliceFlagSeparator = ","
+
 var (
 	slPfx = fmt.Sprintf("sl:::%d:::", time.Now().UTC().UnixNano())
 
@@ -378,5 +380,5 @@ func flagFromEnvOrFile(envVars []string, filePath string) (value string, fromWhe
 }
 
 func flagSplitMultiValues(val string) []string {
-	return strings.Split(val, ",")
+	return strings.Split(val, defaultSliceFlagSeparator)
 }