YITH WooCommerce Stripe Premium 3.20.0

* New: support for WordPress 5.3
* New: support for WooCommerce 3.8
* New: support for API 2019-11-05
* Update: Plugin framework
* Update: StripePHP library
* Tweak: Added check over instance for webhooks that handle disputes
* Tweak: print success message when update_customer method does not find instance match
* Tweak: avoid using deprecated function to retrieve cart url
* Fix: added wrapper function yith_wcstripe_get_cart_hash to provide $cart->get_cart_hash() function in WC
* Dev: reviewed get_customer method to correctly handle exceptions

* New: support to 2019-10-08 API
* Update: internal plugin framework
* Update: Stripe PHP library to version 7.3.1
* Update: Dutch language
* Fix: cards are now stored when processing a trialing subscription
* Fix: possible notice during pay_renew process

* Update: Italian language
* Update: Dutch language
* Tweak: prevent error when $order doesn't exists
* Fix: issue with sync_tokens on customer_updated webhook
* Fix: new check over API error to prevent possible Uncaught exception fatal error
* Fix: enabled flag on renew_needs_action email
* Fix: check used to know if plugin should send renew_needs_action email
* Fix: retrieving order again before sending renew_needs_action email, to double check for correct status
* Dev: added action yith_wcstripe_before_refresh_intent