Checker for the available disk space

For my project, I've implemented checker for the available disk space. Configuration looks like this:
```php
\App\Libraries\SelfDiagnosis\Checks\AvailableDiskSpace::class => [
    'paths' => [
        '/' => '100G', // At least 100G should be available for the path "/"
        '/var/www' => '5G',
    ],
],
```
If project maintainers will be interested, I can send PR