more ways to "dim" the other windows

[gonewest818]

Especially with the addition of the :background and :both modes, there's an opportunity to add more choice in what "dimming" actually does.

Currently the dimmer does a “reduced contrast” adjustment by moving the foreground and background colors closer together. It would also be possible to make the foreground and background “darker”, or make everything “lighter”. This would require yet another configuration variable and some documentation and screenshots to explain.

Another approach would be to grant the user a choice of "target color" which could be any shade and any hue, not limited to black or white. See discussion in #31 for detail.