Resolves #52.

As pointed out by @yorickpeterse, the way vim highlights diffs isn't the most intuitive when you're comparing only two files (vim has to do it this way of course, because it's the only way that makes sense when you're comparing more than two files). When comparing old state on the left with new state on the right, what vim highlights as an addition on the left, actually represents a deletion.

This PR adds a new option enhanced_diff_hl that attempts to tackle this issue. When enabled, a couple new highlighting groups will be generated based on your colorscheme. These will be applied as local winhl in the diff buffers.

enhanced_diff_hl                            *diffview-config-enhanced_diff_hl*
        Type: `boolean`, Default: `false`

        Enable/disable enhanced diff highlighting. When enabled, |hl-DiffAdd|
        in the left diff buffer will be highlighted as |hl-DiffDelete|, and
        the delete fill-chars will be highlighted more subtly.

Before:

After: