Add support for changing VAT rates

[telmich]

As far as I can see from the code, it does not implement the VAT rate change from Germany (DE), which was 16% in 2020: https://www.avalara.com/vatlive/en/vat-news/german-vat-rate-returns-to-19--1-jan-2021.html

Is there any concept / rules in the code base you'd like us to apply in case we were patching the module to support varying rates over time?

[JanmanX]

Hi @telmich,

We would be thrilled to see your patches and improvements fix this module. We do not have any formal concepts / rules for the code in this package. As long as the code is readable and reasonably tested, we should have no trouble merging it in :)

Feel free to reach out to us again, and sorry for the delayed answer!