Allow to specify path to .php_cs file using --config-file option

This PR adds the possibility to specify the path to  the `.php_cs` file using a newly added `--config-file` option.

This is very useful if you have a Git `pre-commit` hook similar to the one suggested by [ZF2](https://github.com/zendframework/zf2/blob/master/README-GIT.md#pre-commit-hook-optional), but want to keep the configuration separate from the hook:

```php
#!/usr/bin/env php
<?php

$changes = [];

exec(
    'git diff --cached --name-status --diff-filter=ACM',
    $changes
);

$exit = 0;

array_walk($changes, function ($change) use (&$exit) {

    if ('D' === substr($change, 0, 1)) {
        return;
    }

    $name = trim(substr(
        $change,
        1
    ));

    $extension = pathinfo(
        $name,
        PATHINFO_EXTENSION
    );

    if (!preg_match('/^ph(p|tml)$/', $extension)) {
        return;
    }

    $output = [];
    $return = 0;

    exec(
        sprintf(
            'php -l %s',
            escapeshellarg($name)
        ),
        $output,
        $return
    );

    if (0 != $return) {

        echo sprintf(
            'Failed parsing: %s: ' . PHP_EOL,
            $name
        );

        echo implode(PHP_EOL, $output) . PHP_EOL;

        $exit = 1;
        return;
    }

    $output = [];
    $return = 0;

    exec(
        sprintf(
            'vendor/bin/php-cs-fixer fix --verbose --config-file=.php_cs %s',
            escapeshellarg($name)
        ),
        $output,
        $return
    );

    if (0 != $return) {

        echo sprintf(
            'Fixed coding style issues in : %s' . PHP_EOL,
            $name
        );

        echo implode(PHP_EOL, $output) . PHP_EOL;

        $exit = 1;
        return;
    }
});

exit($exit);
```

Fixes #251.