Create Command action

You may want to run just a Linux command, for that reason there is Command action.
```php
$command = new Command('whoami');
```

Using Command action you can define arguments as second param:
```php
$command = new Command('cp', array('/path/to/source', '/path/to/destination'));
```

If you prefer arguments can be defined by a key => value array:
```php
$command = new Command(
    'find',
    array(
        '/path/to/dir',
        '-name' => '*',
        '-type' => 'f',
    )
);
```

Command action is based on Callback action so you can also bind triggers for events.