#255: Added a dark-background highlight.  

It isn't perfect, but it is better.  

Here's it on zenburn:
![screen shot 2014-09-29 at 13 49 43](https://cloud.githubusercontent.com/assets/545689/4441817/71fdc048-47d7-11e4-9d29-3f35cf4560ab.png)

And on solarized-dark:
![screen shot 2014-09-29 at 13 49 43](https://cloud.githubusercontent.com/assets/545689/4441820/7b482f3a-47d7-11e4-9597-53af383278c0.png)

I chose the colours from http://raebear.net/comp/emacscolors.html (I did try hex values, but that didn't seem to work in xterm-256color)