Bulgaria IBAN parsing rules

Examples

    iex>  %IbanEx.Iban{
    ...>    country_code: "BG",
    ...>    check_digits: "80",
    ...>    bank_code: "BNBG",
    ...>    branch_code: "9661",
    ...>    national_check: nil,
    ...>    account_number: "1020345678"
    ...>  }
    ...>  |> IbanEx.Country.BG.to_string()
    "BG 80 BNBG 9661 1020345678"