def coverimage
      return nil unless config['coverimage']
      @contents.each do |item|
        if item.media.start_with?('image') && item.file =~ /#{config['coverimage']}\Z/
          return item.file
        end
      end
      nil
    end
def coverimage
      return nil unless config['coverimage']
      @contents.each do |item|
        if item.media.start_with?('image') && item.file =~ /#{config['coverimage']}\Z/
          return item.file
        end
      end
      nil
    end
Construct producer object.
 +config+ takes initial parameter hash. This parameters can be overriden by EPUBMaker#load or EPUBMaker#merge_config.
 +version+ takes EPUB version (default is 2).