feat: add ListById function and test

[alegrey91]

Fix #95

[squeed]

ruleId should be one-indexed to match the iptables binary.

[alegrey91]

Hi @squeed, thanks for your reply.
So, you mean something like this?

// List rules in specified table/chain
func (ipt *IPTables) ListById(table, chain string, Id int) (string, error) {
	args := []string{"-t", table, "-S", chain, strconv.Itoa(Id)}
	rule, err := ipt.executeList(args)
	if err != nil {
		return "", err
	}
	return rule[0], nil
}

[alegrey91]

I think could be more appropriate the name ListById() for this function, right?

[squeed]

This looks good. One minor nit: Id should be lower-cased. Once that's fixed we can merge :-)

[alegrey91]

Done