Add additional attributes to icon extension

[alexander-schranz]

Now not only the class can be set of the icon also accessibility attributes e.g.:

{{ get_icon('test', { role: 'none' }) }}

<span class="icon icon-test" role="none"></span>

Also the service registration of the icon twig extension has been simplified by using and will default font implementation:

services:
    Sulu\Twig\Extensions\IconExtension: ~