[DomCrawler] Added support for 'link' tags in the Link class

| Q             | A
| ------------- | ---
| Bug fix?      | No
| New feature?  | Not really
| BC breaks?    | No
| Deprecations? | No
| Tests pass?   | Yes (at least those of DomCrawler)
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

My use case is that I crawl web pages for RSS links so I need to retrieve the 'href' attributes of 'link' elements.
I would like to use the Link class to resolve encountered 'href' relative URLs.

Please note that I did not replace `if ('a' !== $node->nodeName && 'area' !== $node->nodeName)` with `if (!$node->hasAttribute('href'))` to ensure backward compatibility.
Indeed, according to http://www.w3.org/TR/html5/links.html, the 'href' attribute is optional for 'a' and 'area' elements.