YITH WooCommerce EU VAT Premium 2.31.0

* New: support WooCommerce 3.7
* New: added new field in the user account to store the VAT number
* New: Added filter 'ywev_vat_already_validated_and_valid' to validate extra fields before apply VAT exemption.
* Tweak: replacing magic methods by getters and setters
* Tweak: New parameters to filters ywev_vat_already_validated_and_valid and ywev_vat_validation_response
* Tweak: added the VAT Number field in the user edit page on admin side
* Update: updated plugin core
* Fix: fixed php warnings in the code
* Dev: added plugin upgrade submodule

* New: support to WooCommerce 3.6.0 RC 1
* Update: Italian language
* Update: updated Plugin Framework
* Fix: fixed the shipping tax displayed in the order
* Fix: Correct the tax value in the cart
* Fix: fixing the shipping tax included in the total

New: added the fee taxes to the tax exemption
Tweak: added a new condition to check the EU VAT number
Update: Updated Dutch translation
Update: updated Plugin Framework
Fix: prevent issue (notice and vat discount) when vat number is not set at checkout page
Fix: fixing a possible negative VAT in the order
Fix: fixing warnings in the order metabox
Dev: added a new filter in the ywev-checkout enqueue script