YITH WooCommerce PDF Invoice 4.25.0

* New: support WooCommerce 3.7
* Update: updated Italian language
* Update: updated plugin core
* Fix: fixed the automatic packing slip generation
* Fix: fixed the bulk regenerate documents feature
* Fix: avoid additional tax rows in the total if the tax is zero
* Dev: new filter to change the VAT field name in the checkout
* Dev: fixed a warning with WC 3.7
* Dev: added a new filter ywpi_allow_attach_credit_note
* Dev: changed the method yith_get_prop to get_post_meta in the replace_customer_details_pattern method

* Fix: notice on undefined index
* Dev: new filter 'yith_ywpi_reset_year_invoice_number'
* Dev: new filter 'yith_ywpi_reset_year_document_note_number'
* Tweak: allow placeholders in the invoice notes
* Update: plugin fw
* Update: main language file

* Fix: mandatory SSN field for electronic invoice module