YITH Google Product Feed for WooCommerce 1.31.0

* Update: Spanish language
* Update: Plugin-fw
* Dev: filter yith_wcgpf_add_google_product_feed_information
* Dev: action yith_wcgpf_template_product_information
* Dev: action yith_wcgpf_template_variation_information

* New: Support to WooCommerce 3.6.0 RC2
* Tweak: Add a check control if product is an instanceof WC Product
* Tweak: Added custom label for simple products and variations product
* Update: Spanish language
* Dev: Filter yith_wcgpf_get_product_permalink
* Dev: Filter yith_wcgpf_product_ids

* New: German translation thanks to Alexander Cekic
* Tweak: Improve speed to generate the file
* Dev: Filter yith_wcgpf_product_ids
* Dev: Filter yith_wcgpf_preg_replace_description