Usage:

  class Blog < ActiveRecord::Base
    rendered_column :body, :use_erb=>true
    rendered_column :summary, :format=>'markdown' 
  end

By default, this will try to save the resulting HTML in the
AR fields 'rendered_body' and 'rendered_summary'. You can 
override that by setting :prefix and :postfix. Defaults are:

  :prefix=>'rendered_'
  :postfix=>'',
  :format=>'textile'
  :use_erb=>false