Added Crosshair

Crosshair (or more formally, reticle)
https://en.wikipedia.org/wiki/Reticle. Put it in plugins, because it
seems like an appropriate spot for it.

Made this for my pet project. It would be nice to get it in the main repo.

![screen shot 2013-12-14 at 8 11 17 am](https://f.cloud.github.com/assets/3374510/1748508/858bddd6-64c3-11e3-831f-f532b4513a8d.png)
![screen shot 2013-12-14 at 8 12 00 am](https://f.cloud.github.com/assets/3374510/1748509/8a013a78-64c3-11e3-92c7-4c4bafe1decc.png)