Extended filters in WooCommerce_PDF_Invoices_Export::get_order_items()

Added `order` argument to filters `wpo_wcpdf_order_item_data` and `wpo_wcpdf_order_items_data`. Having the order is necessary to put the item information in context.
Merge pull request #3 from aelia-co/master

Extended filters in WooCommerce_PDF_Invoices_Export::get_order_items()