YITH WooCommerce Membership Premium 2.20.0

* New: support for WooCommerce 3.9
* Update: plugin framework
* Update: language files
* Fix: WPML integration
* Fix: issue when adding products to order on backend
* Tweak: improved style
* Tweak: fixed issue when global variable $post is empty in combination with Gutenberg and Yoast SEO

Update: plugin framework

* New - support to WooCommerce 3.7
* New - add CSS classes to body tag based on the memberships of the user
* Update - plugin framework
* Update - language files
* Fix - issue when overriding Multi Vendor options
* Fix - prevent issue if for some reasons the Shop Manager role doesn't exist
* Fix - duplicate select2 in Gutenberg editor for protected links
* Fix - issue with credits on membership activation