Refactor to make it easier to add new formats

[aaronpk]

Adding a new format should require only adding a new Format class and declaring it somewhere. Currently it requires changing a few files because of the way the URL matching works.

[Zegnat]

Brain storm following.

I think it makes sense to say that the XRay library depends on these Format classes, so passing them at construct time feels right:

// Only let XRay parse Hackernews and XKCD:
$xray = new \p3k\Xray([
    new \p3k\XRay\Formats\Hackernews,
    new \p3k\XRay\Formats\XKCD,
]);
// Should result in an empty output as no parsers work:
$output = $xray->parse('https://example.com');

If no list of formats is provided, it could use some sort of default internal list. If the default list is exposed publicly as a static, users who only want to add a format could retrieve it prior to instantiating XRay:

$formats = array_merge(
    \p3k\Xray::default_formats(),
    [ new \app\XRayFormats\Example ]
);
// Use all default formats plus my own:
$xray = new \p3k\Xray($formats);

This also means there is no need for the formats to be in any specific folder. When XRay is included as a dependency through composer, people will not have to dig through the vendor directory looking for formats.

Another nice thing is that, currently, XRay does some array juggling to find credentials for different APIs. If formats are actual objects they can also have constructors requiring this information. XRay does not need to know about which formats require what authentication, it is up to the formats themselves:

// Parse YouTube through the API:
$xray = new \p3k\Xray([
    new \app\XRayFormats\YouTube($config['apikey'])
]);

[aaronpk]

I made some progress on this in this commit 989d42a, just in terms of now more of the logic is funneled through the same place.