Allow configuration of model classes

This adds the ability to specify the picture and attachment file model classes in the setup block, instead of having to use `Ckeditor::Picture` and `Ckeditor::AttachmentFile`.

A block is accepted so the class does not need to be loaded at configuration time, which makes life easier when other configuration must be done before the class can be loaded. For example, Dragonfly must be configured before my model classes are loaded so that the Dragonfly accessor can be created, but by default the Dragonfly initialization is done in `config/initializers/ckeditor_dragonfly.rb` _after_ the main `Ckeditor.setup` block in `config/initializers/ckeditor.rb`.