YITH WooCommerce Frequently Bought Together Premium 1.39.0

* New: Support to WooCommerce 3.6.0 RC1
* Update: Spanish language
* Update: Plugin Core
* Dev: New hook 'yith_wfbt_end_item'
* Dev: New filter 'yith-wfbt-coupon-individual-use'
* Tweak: Form additional text wrapped inside a tag 'p'

New: Support to WooCommerce 3.5.4.
Update: Plugin Core.
Fix: Percentage discount validation process.
Fix: Integration issue with YITH WooCommerce Multi Vendor.

* New: Support to WooCommerce 3.5.2.
* New: Support to WordPress 5.0.0.
* New: Support to Gutenberg.
* Update: Plugin Core.
* Update: Languages files.
* Update: Dutch translation.