Add media=print stylesheet

[BjarniRunar]

It would be nice to have a stylesheet which makes as many pages as possible print nicely. Probably just something that hides some of the navigation stuff and nukes background colors?

(this came up during the London workshop)

[michaelmior]

This gives something reasonable. There's still clipping in some views, but it's much better than using the existing styles.

#sidebar, #header { display: none }
#content { margin: 0 }
.thread-item-text { clear: both }