Add config option for Arc.Storage.Local directory

[jbrowning]

This PR introduces a local_dir config option that allows one to specify a prefix for the Arc.Storage.Local location. This is especially useful for running tests.

Example usage:

# config/test.exs
config :arc, local_dir: "test/uploads"

defmodule MyApp.Avatar.Uploader do
  use Arc.Definition
  use Arc.Ecto.Definition

  def __storage, do: Arc.Storage.Local

  def storage_dir(_version, {_file, _scope}) do
    "avatars"
  end
end

MyApp.Avatar.Uploader.store(%Plug.Upload{filename: "file.png", path: "/a/b/c"})
# File will be stored in "test/uploads/avatars"

[merqlove]

nice one pr

[jerodsanto]

I'd like to add my hardy 👍 to this feature. For changelog.com we do a similar thing so that we can have Arc upload to priv/uploads in dev and to /uploads in prod (we store files outside app directory and have nginx server them).

Then we do this in our uploaders:

def storage_dir(_version, {_file, scope}) do
  "#{Application.fetch_env!(:arc, :storage_dir)}/#{scope.podcast.slug}/#{scope.slug}"
end

Having this supported out of the box would be super cool.