Implement spoiler tags (fixes #31)

The hit target to expand/shrink is kind of small, it may need some tweaking. Looks like this:

https://www.dropbox.com/s/z8jgo720q39sequ/Screenshot%202014-02-23%2002.04.56.png
https://www.dropbox.com/s/4s55lidsaf0he65/Screenshot%202014-02-23%2002.05.22.png

Visual design recommendations (or code!) generally welcome :)