Added Existence Block Section.

It's pretty much an inverted-inverted section. Similar to the block section, it opens with `{{?foobar}}` and closes with `{{/foobar}}`.

Solves this problem: https://github.com/defunkt/mustache/issues/47

Not sure if you want to pull this in as it's not a part of the official spec (although it should be), but making the request as it would make the life of so many developers easier.