Command runner

This PR will solve #57 

@mnapoli I still need to change the `exec` call in `\Couscous\Step\Scripts\ExecuteScripts::executeScript` could you help with that please?
You don't need to create a `CommandRunner` here and inject it to the deployer.

Rather have it injected in the constructor of `Deployer`, just like for `Filesystem`:

```php
    public function __construct(Filesystem $filesystem)
```

becomes

```php
    public function __construct(Filesystem $filesystem, CommandRunner $commandRunner)
```

The container will inject that automatically.