extend plugin and plugin info to allow for extending models

fixes #1014

Example: 
```
module ConcertoCustomPlugin
  class Engine < ::Rails::Engine
    isolate_namespace ConcertoStatistics
    engine_name "statistics"

    def plugin_info(plugin_info_class)
      @plugin_info ||= plugin_info_class.new do
          extend_model(Screen, ConcertoCustomPlugin::Concerns::AddCustomizationToScreen)
        end
      end
    end
  end
end
```

```
module ConcertoCustomPlugin
  module Concerns
    module AddCustomizationToScreen
      extend ActiveSupport::Concern
      included do
        has_many :things, :class_name => 'ConcertoStatistics::Thing', :dependent => :destroy
      end
    end
  end
end
```