Added post_filters plugin

Plugins that extend PostFilter will get three
additional methods: pre_render, post_render and
post_write. These will be called when every page
or post is processed and allows for the plugin to
modify the post/page object directly to modify
the output beyond what can be done with just
markup.

plugins/post_filters.rb

@@ -0,0 +1,176 @@
+module Jekyll
+
+  # Extended plugin type that allows the plugin
+  # to be called on varous callback methods.
+  #
+  # Examples:
+  # https://github.com/tedkulp/octopress/blob/master/plugins/post_metaweblog.rb
+  # https://github.com/tedkulp/octopress/blob/master/plugins/post_twitter.rb
+  class PostFilter < Plugin
+
+    #Called before post is sent to the converter. Allows
+    #you to modify the post object before the converter
+    #does it's thing
+    def pre_render(post)
+    end
+
+    #Called after the post is rendered with the converter.
+    #Use the post object to modify it's contents before the
+    #post is inserted into the template.
+    def post_render(post)
+    end
+
+    #Called after the post is written to the disk.
+    #Use the post object to read it's contents to do something
+    #after the post is safely written.
+    def post_write(post)
+    end
+  end
+
+  # Monkey patch for the Jekyll Site class. For the original class,
+  # see: https://github.com/mojombo/jekyll/blob/master/lib/jekyll/site.rb
+  class Site
+
+    # Instance variable to store the various post_filter
+    # plugins that are loaded.
+    attr_accessor :post_filters
+
+    # Instantiates all of the post_filter plugins. This is basically
+    # a duplication of the other loaders in Site#setup.
+    def load_post_filters
+      self.post_filters = Jekyll::PostFilter.subclasses.select do |c|
+        !self.safe || c.safe
+      end.map do |c|
+        c.new(self.config)
+      end
+    end
+  end
+
+  # Monkey patch for the Jekyll Post class. For the original class,
+  # see: https://github.com/mojombo/jekyll/blob/master/lib/jekyll/post.rb
+  class Post
+
+    # Copy the #write method to #old_write, so we can redefine #write
+    # method.
+    alias_method :old_write, :write
+
+    # Write the generated post file to the destination directory. It
+    # then calls any post_write methods that may exist.
+    #   +dest+ is the String path to the destination dir
+    #
+    # Returns nothing
+    def write(dest)
+      old_write(dest)
+      post_write if respond_to?(:post_write)
+    end
+  end
+
+  # Monkey patch for the Jekyll Page class. For the original class,
+  # see: https://github.com/mojombo/jekyll/blob/master/lib/jekyll/page.rb
+  class Page
+
+    # Copy the #write method to #old_write, so we can redefine #write
+    # method.
+    alias_method :old_write, :write
+
+    # Write the generated post file to the destination directory. It
+    # then calls any post_write methods that may exist.
+    #   +dest+ is the String path to the destination dir
+    #
+    # Returns nothing
+    def write(dest)
+      old_write(dest)
+      post_write if respond_to?(:post_write)
+    end
+  end
+
+  # Monkey patch for the Jekyll Convertible module. For the original class,
+  # see: https://github.com/mojombo/jekyll/blob/master/lib/jekyll/convertible.rb
+  module Convertible
+
+    def is_post?
+      self.class.to_s == 'Jekyll::Post'
+    end
+
+    def is_page?
+      self.class.to_s == 'Jekyll::Page'
+    end
+
+    def is_filterable?
+      is_post? or is_page?
+    end
+
+    # Call the #pre_render methods on all of the loaded
+    # post_filter plugins.
+    #
+    # Returns nothing
+    def pre_render
+      self.site.load_post_filters unless self.site.post_filters
+
+      if self.site.post_filters and is_filterable?
+        self.site.post_filters.each do |filter|
+          filter.pre_render(self)
+        end
+      end
+    end
+
+    # Call the #post_render methods on all of the loaded
+    # post_filter plugins.
+    #
+    # Returns nothing
+    def post_render
+      if self.site.post_filters and is_filterable?
+        self.site.post_filters.each do |filter|
+          filter.post_render(self)
+        end
+      end
+    end
+
+    # Call the #post_write methods on all of the loaded
+    # post_filter plugins.
+    #
+    # Returns nothing
+    def post_write
+      if self.site.post_filters and is_filterable?
+        self.site.post_filters.each do |filter|
+          filter.post_write(self)
+        end
+      end
+    end
+
+    # Copy the #transform method to #old_transform, so we can
+    # redefine #transform method.
+    alias_method :old_transform, :transform
+
+    # Transform the contents based on the content type. Then calls the
+    # #post_render method if it exists
+    #
+    # Returns nothing.
+    def transform
+      old_transform
+      post_render if respond_to?(:post_render)
+    end
+
+    # Copy the #do_layout method to #old_do_layout, so we can
+    # redefine #do_layout method.
+    alias_method :old_do_layout, :do_layout
+
+    # Calls the pre_render method if it exists and then adds any necessary
+    # layouts to this convertible document.
+    #
+    # payload - The site payload Hash.
+    # layouts - A Hash of {"name" => "layout"}.
+    #
+    # Returns nothing.
+    def do_layout(payload, layouts)
+      pre_render if respond_to?(:pre_render)
+      old_do_layout(payload, layouts)
+    end
+
+    # Returns the full url of the post, including the
+    # configured url
+    def full_url
+      self.site.config['url'] + self.url
+    end
+  end
+end