YITH WooCommerce Color and Label Variations Premium 2.8.1

* New: support for WordPress 5.7
* New: support for WooCommerce 5.1
* Update: YITH plugin framework
* Fix: now is possible to set 0 as value for product attributes
* Dev: new filter 'yith_wccl_force_disable_add_to_cart'

* New: product attributes meta key handler
* Update: YITH plugin framework

* New: Support for WooCommerce 4.5.x
* Update: Plugin framework
* Update: Language files
* Tweak: Improved YITH WooCommerce Wishlist compatibility