[Planning] Crypto Currency support

[kipcole9]

With prompting from @eumend the intent is to have clearer support for Crypto Currencies in Money. This issue is to capture requirements and to note functional plans.

First steps

• Create Money.Crypto to represent a crypto currency
• Use https://min-api.cryptocompare.com/data/all/coinlist as the API to return the list of valid crypto currencies.
• Since crypto currencies are much more dynamic that ISO4217 currencies, a GenServer will be used to manage the list of currencies which will be refreshed at some frequency.
• Exchange rates. These change a lot so perhaps they need to be retrieved on demand? Small time window?

[LostKobrakai]

👍 for moving that in a custom module instead of shoehorning it into a currency.

[ryanwinchester]

Is this an optional thing? I don't need a refreshing list of cryptocurrencies or anythjng to do with this.

Should it be a separate lib?

[kipcole9]

You are right - and it will be in a separate lib. Thanks for the sanity suggestion!