I made a pull request.
I’m quite happy with how the feature turned out, hopefully people here agree. Basically, it works the same way as custom syntax sets do now 
Implementation details:
- I added a once_cell dependency to store the extra highlight theme set (a Syntect ThemeSet)
- I moved the extra syntax set out of the Config struct to be a OnceCell too
I’m a little inexperienced when it comes to pull requests. I think mine is a bit messy. I included lots of fixes here and there which I needed to be able to run and pass tests on the next branch.
I also ended up moving the markdown related config stuff marked TODO: 0.14.0 into the Markdown struct. I either did that or added more TODO: 0.14.0 Move to Markdown struct stuff which felt silly 
Would it be better to exclude the not directly related changes to make it easier to review?
Anyway, I was happy to get a chance to do this. My only regret is not starting it sooner.
Since I should have posted here first, have no mercy. Let me know any changes you want/need 