Added some hooks

Loved your plugin, needed extra hooks, here is my code https://gist.github.com/Nakira/827e772d6799b963202b1d0139bda097

It simply adds 2 hooks :
– wc_order_pdf_extra_order : receives the order, allows to add extra lines to pdf
– wc_order_pdf_extra_order_item : receives the order_item, allows to add extra lines to pdf about item