Function for returning the country code should validate

[railsmechanic]

The function which returns the country code for a given IBAN should check whether the given country code of the IBAN is within the list of supported countries:

bankster/lib/bankster/iban.ex

Lines 187 to 191 in 1572aaf

	def country_code(iban) do
	iban
	|> format_default()
	|> String.slice(0..1)
	end

Change from returning a simple string to
{:ok, "DE"} or {:error, :invalid_country}

This is a breaking change!