Add support for multiple preview_path

Hi guys.

I'd like to add support for multiple preview_path of ActionMailer.   (It could configure only one directory path now.)

Because I think I'd like to configure it in Rails Engine. 

For example.

```rb
initializer "spree.core.mailers_previews" do |app|
  app.config.action_mailer.preview_path << File.join(root, "test", "mailers", "previews")
end
```