Google Tag Manager pour WooCommerce PRO 1.18.0

* add - filter `gtm_ecommerce_woo_split_item_list_max_items` to configure max items per view_item_list chunk (default 25)
* tweak - tested against the latest versions of WordPress and WooCommerce

* add - adds `user_data` event on checkout page after filling email/phone fields

* add - adds `user_data` event on all pages if user is logged in

* tweak - tested against the latest version of WordPress

* add - adds fallback for `add_payment_info` and `add_shipping_info` in case of non-standard checkout layouts
* add - adds `user_data` property to `purchase` event
* add - unifies `user_data` event format
* tweak - tested against the latest version of WooCommerce

* add - add `content_type` property to event's items
* tweak - tested against the latest versions of WordPress and WooCommerce

* add - add 'customer_type' property to the 'purchase' event to distinguish between new and returning customers
* add - add 'payment_type' property to the 'purchase' event

* fix - fix issue with 'add_to_cart' event on product page, related to product variants

* fix - fix issue with 'add_to_cart' event on cart page, related to product variants
* tweak - tested against the latest version of WooCommerce

* add - improve conversion tracking monitoring UI and documentation
* tweak - tested against the WooCommerce 10!