add service alias for slugify

In the docs, it says you can access the slugify service via:

```php
$slug = $this->get('slugify')->slugify('Hello World!');
```

This does not work currently - you need to use:

```php
$slug = $this->get('cocur_slugify')->slugify('Hello World!');
```

This PR adds a `slugify` service alias to allow the code in the docs to work.