Austria IBAN parsing rules

Examples

    iex>  %IbanEx.Iban{
    ...>    country_code: "AT",
    ...>    check_digits: "61",
    ...>    bank_code: "19043",
    ...>    branch_code: nil,
    ...>    national_check: nil,
    ...>    account_number: "00234573201"
    ...>  }
    ...>  |> IbanEx.Country.AT.to_string()
    "AT 61 19043 00234573201"