Add config option to change folded line bg

[rux616]

I love the colorscheme, but I found that sometimes I would lose track of code folds as they blend into the background. So I decided to add a configuration option (badwolf_folded) to adjust the background color in a similar manner to the tabline. It defaults to no change, so existing users won't be affected negatively.

Side-by-side-by-side-by-side image for comparison (badwolf_folded values are 0, 1 [default], 2, and 3, respectively): https://imgur.com/a/LyMxdAI