Added Stack Exchange-style spoiler tags

Over on Stack Exchange, they implemented spoiler tags this way:

    >! spoiler!

Which would get rendered as:

    <blockquote class="spoiler">spoiler!</blockquote>

Built-in spoiler markdown is a popular request on reddit, and I think this'll make spoilers much nicer to use on reddit (especially on mobile). This won't support inline spoilers, though; some other syntax will need to be implemented.

In a commit to `snudown`, I committed the changes to the parser.