#35 Allow changing color of links

[fritzophrenic]

Default colors of links in HTML can conflict with background color, as noted on Vim's github issues: https://github.com/vim/vim/pull/10191

The pull request just inherits the color from the surrounding text, it would be good to default to that but also allow specifying a color (or maybe a full style?) in case the link needs to stand out from the text.