Add Support for markdown.nvim

[avih7531]

I'd like to add support for the rending plugin markdown.nvim -- an awesome, real-time markdown generator for Neovim.

Happy to tackle this one later in the week, but by all means if somebody wants to beat me to it, here are the highlight groups.

Before I start, I would like feedback as to what elements should be which colors? As I'm not a maintainer (happy to become one) I don't provide this project creative direction, and therefore want to make sure that whatever I do truly sticks to @AlexvZyl and @5-pebbles vision. Would love some basic ideas on highlights and color groupings, but will probably make a few and send screenshots before I submit the pull request.