Ability to only preview open folds

Hi,

My use case is that I have a very large markdown file (300,000+ words). Think of it as an entire book in a single file where I have L1 headers as chapters.

I often close all of the folds except for the chapter that I'm actively writing, and Vim is smart enough to only perform a lot of actions on open folds.

It would be amazing if you could add in a feature / option to your plugin so that it only previews open folds. Maybe some type of option like `let g:mkdp_preview_closed_folds = 1`, which acts exactly how it does now, but when set to `0` then only open folders will be previewed.