adding support for specifying a delegate option to imagemagick

This is to allow for custom processing of files through imagemagick using it's built in delegate handling.  We are using this to generate thumbnails from movies

Simple example of use case:
```ruby
class Movie < ActiveRecord::Base
  extend Dragonfly::Model 
  dragonfly_accessor :preview
  dragonfly_accessor :file do 
     copy_to(:preview) do |mov|
        mov.convert("", 'format' => 'jpg', 'frame' => 1, 'delegate' => 'ffmpeg')
     end
  end
end
```