Add support for TaxCurrencyCode (BT-6)

[andras-minicrm]

Hi @josemmo

First: thanks for this awesome library!

I'm currently developing integration with anaf (romanian tax authority). They use a special ruleset: RO_CIUS
They have a special (fatal) rule:
BR-RO-030: If the "Invoice Currency Code" (BT-5) is other than "RON", then the item "VAT Accounting Currency Code" (BT-6) must be "RON".

It seems the library doesn't have a support for BT-6. We've created two xml-s with smartbill I've attached them here. However BT-6 requires an extra:

<cac:TaxTotal>
    <cbc:TaxAmount currencyID="RON">93.89</cbc:TaxAmount><!--BT-111-->
</cac:TaxTotal>

so BT-6 maybe needs an exchange rate option to correctly calculate the amount (eg: EUR to RON: 4.1234, RON to RON 1.000)?

Samples:
RON2RON.txt
EUR2RON.txt

(these are xmls but github doesn't allow xml uploads)

[josemmo]

Hello @andras-minicrm,

Thanks for the feedback! I've implemented the feature, will be merged in the next release.

[andras-minicrm]

Amazing, thx!! When the next release scheduled?

[josemmo]

We don't have a scheduled release cycle, the next release will be (probably) in June.