Add blackhole storage

This can be useful as a storage in cases when you do not need or want to
store cached images anywhere, for example because you have CDN in front
of the Imager and it is caching the requests.

lib/imager/config/store_transform.ex

@@ -54,7 +54,7 @@ defmodule Imager.Config.StoreTransform do
   defp get_cache(%{cache: values}), do: parse(values)
   defp get_cache(values), do: parse(values)
 
-  defp parse(%{type: type} = values) when type in ~w(S3 Local) do
+  defp parse(%{type: type} = values) when type in ~w(S3 Local Blackhole) do
     module = Module.safe_concat(Imager.Store, type)
     opts = Keyword.new(Map.get(values, :options, []))
 

lib/imager/store/blackhole.ex

@@ -0,0 +1,12 @@
+defmodule Imager.Store.Blackhole do
+  @behaviour Imager.Store
+
+  @moduledoc """
+  `/dev/null` of the stores. All writes will discard data and all reads
+  will automatically fail.
+  """
+
+  def retrieve(_path, _ops), do: :error
+
+  def store(_path, _mime, stream, _opts), do: stream
+end