Add a `Decimal` type

[Krister Viirsaar]

i use Decimal for sensitive numbers and cloak_ecto doesn't seem to support it.

it doesn't look too hard to implement, but can someone check this? seems to be working for me.

defmodule Cloak.Ecto.Decimal do
  defmacro __using__(opts) do
    opts = Keyword.merge(opts, vault: Keyword.fetch!(opts, :vault))

    quote do
      alias Decimal, as: D
      use Cloak.Ecto.Type, unquote(opts)

      def cast(value), do: Ecto.Type.cast(:decimal, value)

      def before_encrypt(value) do
        case Ecto.Type.cast(:decimal, value) do
          {:ok, d} -> D.to_string(d)
          _error -> :error
        end
      end

      def after_decrypt(value) do
        case D.new(value) do
          %D{} = d -> d
          _error -> :error
        end
      end
    end
  end
end