Add ErrorObject.Is method

[goodevilgenius]

Similar to #24, but this adds an Is method to ErrorObject. This allows us, in conjunction with #24, to do something like this:

type Stuff struct {
	Name string
}

func (st Stuff) Validate() error {
	return validation.ValidateStruct(
		&st,
		validation.Field(&st.Name, validation.Lenth(1, 5)),
	)
}

func main() {
	st := Stuff{}
	err := st.Validate()
	fmt.Println(err)
	if errors.Is(err, validation.ErrLengthOutOfRange) {
		fmt.Println("this stuff is the wrong size")
	} else {
		fmt.Println("this stuff is the right size")
	}
}

[swithek]

Looks good, thanks 👍