Add Twig filter to add classes

I've created a second merge request to backport this feature to the 8.x-1.x version. The only difference between this and the 2.0.x version is one line:

new \Twig_SimpleFilter('add_class', [$this, 'addClasses']),

instead of:

new TwigFilter('add_class', [$this, 'addClasses']),

The addClasses function is the same.

rlhawk → closed merge request !3

A new add_class Twig filter was just added to Drupal core in ✨ Create twig filters: |add_class and |set_attribute Fixed , which means this work need to be reverted, unfortunately. Perhaps we can come up with a new name for a class-specific filter provided by this module.