def write filename, options={}
      extension = file_extension(filename)
      send("write_to_#{extension}".to_sym, filename, options)
    end
def write filename, options={}
      extension = file_extension(filename)
      send("write_to_#{extension}".to_sym, filename, options)
    end
Writes the book to a file. Filetype is based on the extension, but can be overridden

 @param [String] filename   a string with a reference to the file to be written to
 @param [Hash] options  depends on the writer chosen by the file's filetype