Add CustomVerifier#and & CustomVerifier#andComparable methods to allow even more fluid chaining

[neocotic]

This PR adds complimentary methods for Verifier#verify and Verifier#verifyComparable to CustomVerifer as and and andComparable to make chaining more fluid. Here's the example from the README:

package com.example.form;

import io.skelp.verifier.Verifier;

public class LoginForm implements Form {

    UserService userService;

    @Override
    public void handle(Map<String, String> data) {
        Verifier.verify(data)
            .containAllKeys("username", "password");
            .and(data.get("username"), "username")
                .not().blank();
            .and(data.get("password"), "password")
                .not().empty()
                .alphanumeric();

        userService.login(data);
    }
}