Cache converter in renderer (#7183)

Merge pull request 7183

lib/jekyll/renderer.rb

@@ -96,6 +96,7 @@ module Jekyll
     #
     # Returns String the converted content.
     def convert(content)
+      Jekyll::Cache.new("Jekyll::Renderer").getset(content) do
         converters.reduce(content) do |output, converter|
           begin
             converter.convert output
@@ -108,6 +109,7 @@ module Jekyll
           end
         end
       end
+    end
 
     # Render the given content with the payload and info
     #