add service alias for slugify

[kbond]

In the docs, it says you can access the slugify service via:

$slug = $this->get('slugify')->slugify('Hello World!');

This does not work currently - you need to use:

$slug = $this->get('cocur_slugify')->slugify('Hello World!');

This PR adds a slugify service alias to allow the code in the docs to work.