Ability to add more tax/GST slabs.#445

Hi
Your application is very good and performance is too good.
Here we would kie to suggest that application should support more tax slabs. Currently it supports only two tax slabs only. In real life we need to have more than 2 slabs in one invoice.
In india we have 4 to 5 GST slabs. So in one invoice we can have 4 itesh having 4 different tax rates.

It will be good to have more tax slabs in our application.

Thanks 😊