Don't highlight +/- columns in patches (configurable)

The highlighting of the +/- column was bothering me to the point where I turned it off completely.  You often see patches when using vim-fugitive.  So this will allow you to configure filetypes where the first and second column won't get highlighted.

I use the color link to switch to faint colors in sessions where it doesn't matter and to toggle between colors.  I think it's useful, too.