Enhance colors of different heading-levels

[BenjaminBrandtner]

Right now this plugin defines every heading (a line starting with one or more #) as the highlight group notesAtxHeading.

Many colorscheme actually include different colors for different levels of headings inside markdown files. To do that they style the highlight groups markdownH1 to markdownH6 as they are defined in the markdown syntax file that is shipped with vim.

I don't know if it would break anything, but I think if you additionally defined these standard highlight groups in your syntax file, one could make use of such colorschemes that style them differently, which looks better in my opinion.

Editing a Markdown file with colorscheme gruvbox:

Editing a Notes file with colorscheme gruvbox: