Add support for rdiscount extensions

Specify extensions at your _config.yml file:

    ...
    rdiscount:
        extensions: [smart, autolink]

Available extensions can be found here:
http://rdoc.info/projects/rtomayko/rdiscount

closes jekyll#173

lib/jekyll.rb

@@ -71,6 +71,9 @@ module Jekyll
       'png_engine' => 'blahtex',
       'png_dir'    => 'images/latex',
       'png_url'    => '/images/latex'
+    },
+    'rdiscount'    => {
+      'extensions' => []
     }
   }
 

lib/jekyll/converters/markdown.rb

@@ -13,6 +13,9 @@ def setup
         when 'rdiscount'
           begin
             require 'rdiscount'
+
+            # Load rdiscount extensions
+            @rdiscount_extensions = @config['rdiscount']['extensions'].map { |e| e.to_sym }
           rescue LoadError
             STDERR.puts 'You are missing a library required for Markdown. Please run:'
             STDERR.puts '  $ [sudo] gem install rdiscount'
@@ -67,7 +70,7 @@ def convert(content)
       setup
       case @config['markdown']
         when 'rdiscount'
-          RDiscount.new(content).to_html
+          RDiscount.new(content, *@rdiscount_extensions).to_html
         when 'maruku'
           Maruku.new(content).to_html
       end

test/test_rdiscount.rb

@@ -0,0 +1,18 @@
+require File.dirname(__FILE__) + '/helper'
+
+class TestRdiscount < Test::Unit::TestCase
+
+  context "rdiscount" do
+    setup do
+      config = {
+        'rdiscount' => { 'extensions' => ['smart'] },
+        'markdown' => 'rdiscount'
+      }
+      @markdown = MarkdownConverter.new config
+    end
+
+    should "pass rdiscount extensions" do
+      assert_equal "<p>&ldquo;smart&rdquo;</p>", @markdown.convert('"smart"').strip
+    end
+  end
+end