Another attempt at vim diff highlighting

My alternative to pull request #160 which caused some controversy.

What I did here:
- I restored the foreground colors and just changed the background color to a more subtle color
- I left the colors for the "diff" filetype as they originally were
- I gave 'DiffText' a different foreground color so that it is more obvious which part of the line was changed

In particular, I'm not sure about the last point. I only use 'ocean' and don't know if this works well with other schemes - I'm not an expert on base16 (or even color schemes at all). Also, I've found that the color chosen here affects the background for the normal mode indicator when using the Airline plugin.