Add function to fetch valid currency codes

[ohjakobsen]

If the user provides and invalid currency code to get_exchange_rate(), an error is returned. A list of valid codes are available from a link in the documentation, but this is not very transparent for the user. Since we already provide a list of valid codes internally, we should create a function to make this information available to the user. Such a function could be called list_valid_currencies(). When an error is returned from get_exchange_rate() because of an invalid currency code, we could tip the user to list valid codes with list_valid_currencies().