added support for markdown.nvim | changed some native markup/down highlights TBD if final

[avih7531]

Added support for markdown.nvim

Had to change multiple @markup highlight groups because of preset bolds and italics, to get a nice looking RenderMarkdown, but think it came out great.

Any thoughts on whether or not to keep the bolds for H1-3 and italics for H5-6 permanently? If so, should they persist in the RenderMarkdown groups? My vote would be no as I'm unfamiliar with the convention for highlights like that, but then again, not my repo :)

AH