Add support for the Rouge syntax highlighter

By setting the `highlighter` setting to `rouge` you can now easily
highlight your code with it instead of relying on Pygments. However,
Jekyll doesn't depend on Rouge explicitly, you will need to install it
or add it to your Gemfile.

The documentation has been updated accordingly.

test/test_redcarpet.rb

@@ -42,6 +42,22 @@ puts "Hello world"
       end
     end
 
+    context "with rouge enabled" do
+      setup do
+        @markdown = Converters::Markdown.new @config.merge({ 'highlighter' => 'rouge' })
+      end
+
+      should "render fenced code blocks with syntax highlighting" do
+        assert_equal "<div class=\"highlight\"><pre><code class=\"ruby language-ruby\" data-lang=\"ruby\"><span class=\"nb\">puts</span> <span class=\"s2\">\"Hello world\"</span>\n</code></pre></div>", @markdown.convert(
+          <<-EOS
+```ruby
+puts "Hello world"
+```
+          EOS
+        ).strip
+      end
+    end
+
     context "without any highlighter" do
       setup do
         @markdown = Converters::Markdown.new @config.merge({ 'highlighter' => nil })