[Utilities][Opacity] Add opacity classes

Created by: PululuK

Easily change the opacity of elements with the opacity utilities.

This can be very useful in some context, for example :

<section class="module-list {% if not hook['active'] %} opacity-50 {% endif %}">

Instead of :

<section class="module-list" {% if not hook['active'] %} style="opacity:50%" {% endif %}>

I can provide a PR if you are interested in this feature.