YITH WooCommerce Waiting List Premium 3.9.0

* New: message to show when a guest user subscribes to a waiting list with an email already registered
* Update: Plugin framework

* New: Support to WooCommerce 3.7.0 RC1
* Update: Plugin Core
* Update: Italian language
* Fix: Prevent multiple product save on delete waiting list process
* Fix: Check that variable implements countable before pass it to 'count' function

* New: support to WooCommerce 3.6.4
* New: support to WordPress 5.2
* Update: Plugin Core