Add used template name to default output

[netson]

Hi, first of all thank you for this great package! I have been using it for many years and it has saved me tons of time.

I am currently rewriting my document processing tooling and have a need to use the template name which was used to extract data. I have forked i2d and made the (minor) change in the invoice_template.py file and would love to send a PR to have it included, but I first wanted to ask if this change is a welcome one? If so, I submit a PR.

Keep up the good work! :-)