Validator

>  We should hide this

the reason behind this exported field, is same with `TagName` since we are giving the ability to customize it, i thought it would be better to give the same flexibility for its `value`

I dont have any strong opinion about exporting TagName and TagValue, since they are implementation details. Here i just wanted to be consistent about the API. For that reason if you think we should really hide this, i can use `strconv.ParseBool`