Add `FileSystemStackTask` and `Shared\CommandStack`.

Using `Robo\Task\FileSystem`:

```php
$this->taskFileSystem()->mkdir(['tmp', 'cache'])->run();
$this->taskFileSystem()->touch('.maintenance')->run();
```

Instantiating `Robo\Task\Shared\CommandStack`:

```php
$cli = new Robo\Task\Shared\CommandStack();
$cli->pushCommand('rm -rf ./docs');
$cli->pushCommand('sphinx-build');
$result = $cli->run();
```

Extending `Robo\Task\Shared\ComandStack`:

```php
class SomeExecutabledStackTask extends Robo\Task\Shared\CommandStack
{
    public function __construct($executable = '/path/to/executable')
    {
        $this->setExecutable($executable);
    }
}
```