After plugin

I made a simplest-possible 'After' plugin.

It exposes an `after` method in controllers so you can run some code after everything's generated.

It let me write this in my `controller.rb`, which brought joy:

    after do
    	File.open( "./public/sitemap.txt", "w" ) do |f|
    		Dir["public/**/*.html"].each do |n|
    			f.puts "http://danceliquid.com/#{n}"
    		end
    	end
    end

Take it, if you'd like. (: