Added possibility to define custom fixers by the configuration class

This address Issue https://github.com/fabpot/PHP-CS-Fixer/issues/186. You can define add a custom fixer to the config class:

<?php

use Symfony\CS\FixerInterface;
use Symfony\CS\Fixer\IndentationFixer;

class UseTabsFixer extends IndentationFixer  implements FixerInterface
{
	public function fix(\SplFileInfo $file, $content)
	{
		return preg_replace_callback('/^( {4})/m', function ($matches) use ($content) {
			return str_replace('    ', "\t", $matches[0]);
		}, $content);
	}
	public function getName()
	{
		return 'use_tabs';
	}
	public function getDescription()
	{
		return 'Code must use tabs for indenting.';
	}
}

$finder = Symfony\CS\Finder\DefaultFinder::create()
	->in(__DIR__);

$config = Symfony\CS\Config\Config::create()
	->fixers(array(
		'linefeed',
		'trailing_spaces',
		'php_closing_tag',
		'controls_spaces',
		'use_tabs'))
	->finder($finder);

$config->addCustomFixer(new UseTabsFixer());

return $config;