Added all SEPA countries (IBANs from European Union countries) and minimal test coverage for countries

lib/iban_ex/country/dk.ex

@@ -1,6 +1,13 @@
 defmodule IbanEx.Country.DK do
   @moduledoc """
   Denmark IBAN parsing rules
+
+  ## Examples
+
+    iex> %IbanEx.Iban{country_code: "DK", check_digits: "50", bank_code: "0040", branch_code: nil, national_check: nil, account_number: "0440116243"}
+    iex> |> IbanEx.Country.DK.to_string()
+    "DK 50 0040 0440116243"
+
   """
   @size 18
   @rule ~r/^(?<bank_code>[0-9]{4})(?<account_number>[0-9]{10})$/i