new extension: spoilerblock

In the HTML renderer, a block prefixed with `?>` will be wrapped in
<div class="spoilers">...</div>.  Using style sheets this block could
then be hidden by default.

Example:

  I really liked this movie.

  ?> But at the end everybody died.
  ?> What a pity.
  ?> You don't have to read this because it will
  ?> be hidden by default.