Worldwide tax calculation option

[MarkCanada]

With invoice immutability being pushed on us, we really need a way to properly calculate taxes before invoices are published and can't be changed any more.

There are multiple areas in the world where WHMCS's tax rules are too limiting, but there is no tax calculation hook where addons can overwrite the incorrect tax calculations. 

A simple hook that we can use after WHMCS calculated its taxes (for orders and invoices) where we can overwrite the tax calculations is all that is needed so tax compliance is not limited as much by WHMCS any more.

As a user, if you need to have taxes calculated differently, please vote at https://requests.whmcs.com/idea/hook-for-tax-calculation_1 to convince WHMCS to implement a way to have the option to have any taxes you need calculated by an addon at least.

WHMCS: This shouldn't take too long to program and document, and if you only have one new subscriber because they can use your system now, it'll pay for itself.