Add ability to register custom reporters

[llaville]

Hello,

Because I've worked on two reports (an improved version of standard txt format, and a new sarif format), I'm a bit sad to see that there is no ability yet to register it as it's case for custom fixers.

So today, I've worked on it, and I'm able to propose a solution (already available on my fork : see https://github.com/llaville/PHP-CS-Fixer/tree/features/add_custom_reporters)

This new feature is summarized like that : https://github.com/llaville/PHP-CS-Fixer/blob/features/add_custom_reporters/doc/custom_reports.rst

E.g, may be invoke as standard command vendor/bin/php-cs-fixer check src/SarifLog.php --format sarif

With a .php-cs-fixer.php like this :

<?php

return (new PhpCsFixer\Config())
    ->registerCustomReporters([
        new \Bartlett\Sarif\Converter\PhpCsFixerConverter(),
    ])
    ->setFinder(
        PhpCsFixer\Finder::create()
            ->files()
            ->in(__DIR__ . '/src')
            ->name('SarifLog.php')
    );

WDYT about this new feature idea ?