#361 Adding the ability to use absolute domain

[nyamsprod]

Resolve issue #361 by adding:

• DomainName::withRootLabel();
• DomainName::withoutRootLabel();
• DomainName::isAbsolute();

The resolution API does not change but the resolve now can also resolve absolute domain:

<?php

$result = $publicSuffixList->resolve('private.ulb.ac.be.');
$result->suffix()->toString();                     // returns 'ac.be'
$result->domain()->toString();                     // returns 'private.ulb.ac.be.'
$result->registrableDomain()->toString();          // returns 'ulb.ac.be'
$result->secondLevelDomain()->toString();          // returns 'ulb'
$result->subDomain()->toString();                  // returns 'private'
$result->domain()->isAbsolute();                   // returns true
$result->domain()->withoutRootLabel()->toString(); // returns 'private.ulb.ac.be'