YITH WooCommerce Stripe Premium 3.20.0

* New: WooCommerce 3.6 support
* New: retry renews when a fail occurs
* New: added check over site url, to set Test Mode when plugin is enabled on a staging installation
* Tweak: disabled Make default button for expired cards
* Tweak: improved card form on mobile devices
* Tweak: avoid duplicated cards in card expiration reminder queue
* Tweak: updated card expiration reminder email, to provide correct information when card is already expired
* Tweak: improved error reporting system
* Tweak: system now register card fingerprint during webhook handling
* Update: internal plugin framework
* Update: dutch translation
* Fix: issue when trying to pay with a previously registered card (default card was applied)
* Fix: stripe do not execute code when customer tries to delete/set as default tokens created with other gateways
* Fix: avoid to register token twice, when the same card_id is already saved for the same customer
* Fix: avoid saving cards when related option is disabled

New: WooCommerce 3.5.4 support
New: button to automatically set webhook on Stripe
New: added reminder email for card expiration
Update: internal plugin framework
Update: Stripe php library to version 6.29.3
Update: Dutch language
Fix: processing renew instead of new order when reactivating subscriptions

* New: WordPress 5.0 support
* New: WordPress 3.5.2 support
* New: support to latest API version (2018-11-08)
* New: admin can now choose whether they want to automatically store card reference, or ask customer which cards to save (for appropriate payment modes only)
* Tweak: prevent stripe to execute subscriptions methods, when payment method for the subscription is not stripe
* Tweak: added check over gateway existence before filtering YWSBS from list
* Tweak: improved Webhook error messages
* Tweak: check over configurable properties in update_subscription method
* Update: internal plugin framework
* Update: dutch translation
* Fix: error in js to trigger elements handling
* Fix: solved issue occurring when billing state select is not visible
* Fix: implicit casting when registering a new failed invoice
* Fix: call to undefined endpoint, causing a 404 error in API; Limited labels accordingly to new limit set for the API
* Dev: added filter yith_wcstripe_error_message_order_note to let third party code filter error messages stored in order notes