Feature Request: Add a way to register custom (non-ISO) currencies

[dejurin]

Hi! Thanks for this library. I love using bojanz/currency for ISO-defined currencies and localizing them. However, I occasionally need to work with codes that are not in the standard ISO 4217 list, such as:

• Cryptocurrencies (BTC, ETH, etc.).
• Custom or test codes for internal usage.

Currently, bojanz/currency has no public API to register or define additional currency codes. If I try to do NewAmount("100", "BTC"), it fails with an “unknown currency code” error (since BTC is absent in data.go).

I’d like to request a feature that allows adding custom entries at runtime, similar to how some libraries have a Register or AddCurrency function. This could look like: Then, the code NewAmount("123.45", "BTC") would succeed, and formatter.Format would also work if I supply it with, say, a default symbol or pattern.

Rationale:

1. Many real-world applications mix official ISO currencies with special tokens or private codes.
2. A Register approach would keep the library’s built-in data intact while letting advanced users inject their own definitions.
3. Without it, we either have to fork data.go or maintain a separate logic for custom codes and do manual formatting.

Possible approach:

• A public map or method in the package that merges user-provided data with the built-in CLDR sets.
• For formatting, if a custom code is found, we use user-specified digits/symbol instead of failing.
• Ideally, it could be a single currency.Register(...) method.

I realize that might require some discussion about how to handle locale-specific symbols, but even minimal support (like picking a fallback or “generic” symbol) would be extremely helpful.

Questions:

1. Would you be open to a PR that introduces a Register() or AddCurrency() method?
2. Alternatively, is there a recommended approach/pattern for working with non-ISO codes that I might have missed?

Thanks again for your great work on this library! Let me know if you need more details or if there’s another recommended solution for custom codes.

[bojanz]

Thanks, I'll review this in ~7 days.

[dejurin]

Thank you. 🙏 I hope this works out for you.