Better call closures

If you use space between `function` keyword and the brackets or if you use comments like `// something` Jobby can't recognize function, ie:

ie.:

```php
$jobby->add(
    'foo',
    [
        'command' => function () {
            echo 'foo bar';
            // some comment
            return true;
        }
    ]
);
```

After my PR, you can use single line comments, `functions ()` structure with space between keyword and the brackets and even more: you can now use construct `function () use (...)`, for example:

```php
<?php

// Setup autoloading
require 'init_autoloader.php';

// 'config'
$config = 'test';

$jobby = new \Jobby\Jobby();
$jobby->add(
    'bar',
    [
        'command' => function () use ($config) {
            // $config will be 'test' here

            return true;
        },
        'enabled' => true,
    ]
);

$jobby->run();
```