YITH Google Product Feed for WooCommerce 1.31.0

* New: Support to WooCommerce 3.9.0
* Tweak: prevent fatal error on generate feed file
* Update: Plugin-fw
* Dev yith_wcgpf_get_product_title filter

* Update: Plugin-fw

* Tweak: check isset the variable before print the content
* Tweak: check if isset $values['feed_template']
* Fix: display google product feed variation information
* Fix: generate google categories for en locale
* Update: Italian language