YITH Custom Thank You Page for Woocommerce Premium 3.7.0

* New: support to WooCommerce 3.5.x
* New: support to WordPress 4.9.8
* Update: Plugin Core

* New: Set Custom Thank you Page for Payment method
* New: Pdf Button color Styles
* New: Option to delete options saved in database on plugin uninstall
* Update: Language files
* Dev: added filter yith_ctpw_pdf_file_name (to change the pdf downloaded file name)

* New: Support to WordPress 4.9.8
* New: Save as PDF button shortcode [yith_ctpw_pdf_button]
* Update: YITH Plugin Framework
* Update: Language files
* Fix: custom thank you page for variable product
* Fix: product description for variations products in social box
* Dev: added filter yith_ctpw_url_redirect (to manage redirect url)
* Dev: added filter yctpw_avoid_pages (to remove specific pages from the select page field)