Adds colors for italic, bold, and code in markdown

[mikeknep]

I like @xdl's suggestion in #51, except I don't want code and headers to be the same color. (Some fonts, including Fira Mono which I use, don't have bold defined so that distinction is lost in vim.) Also added bold and italics.

I found that if you don't have Tim Pope's vim-markdown plugin installed, the bold delimiters (should be red) pick up the italic color (yellow). There are also several discussions about inline underscores in his repo—it seems like the best thing to do is just escape them, so I left as-is.

[morhetz]

Hi. I've made some markdown highlighting facelift for gruvbox and I would like to thank you for advice to split markdownCode and markdownHeaders colors. Finally, markdownCode is not blue actually, as I've found aqua works best.
Speaking of bold and italics extra-highlighting you've suggested, that's not the something I would like to merge to master, since result looks a little bit overcolored.
By the way, I'm pretty sure Fira Mono has bold defined, though it lacks italics