Use RedCarpet to Render Markdown

Most of the markdown I write ends up on GitHub, so when I preview my markdown I on my computer I like it to look exactly like it will once I push it live to some repository. I also like to use the excellent Marked app to preview my markdown. Unfortunately, even though Marked claims to support Github Flavored Markdown (GFM) it leaves some features out. I'm not going to blame Marked for the incomplete implementation of GFM, they outsource the processing to Discount, so it's probably not their fault. But certain features of GFM, like nested lists, are really useful so I decided to switch to RedCarpet, since that is the Gem GitHub itself uses to parse READMEs.
So, for anyone else who runs into the issue of Marked's incomplete GFM support, simply install redcarpet and use it as a custom processor:
sudo gem install redcarpet
In the "Advanced" section of Marked's preferences you can find the custom processor section:
Custom Marked Processor
Custom Marked Processor
Note: You might have to click the "Update Permissions" button since Marked may not have read access to wherever you're redcarpet executable is located.
Once you've added the custom processor you should get a little nondescript button near the lower right-hand side of your Marked windows:
Custom processor button in Marked
Custom processor button in Marked
If you click that button you should now get markdown rendered through your custom processor! Nice 😎
Hope that helps anyone who ran into the same problem I did.