Allow unstable features for the unstable book

[leonardo-m]

The home page of the Unstable book shows an example with the box syntax. But if I try to run it in the page, the compilation fails because unstable features are not supported.

[steveklabnik]

The playpen takes a &version=nightly parameter onto its URL, so I believe that if we see #[feature in the code, we should tack that on.

[azerupi]

Considering we now support rust,something syntax. We could also take advantage of that and tag code blocks with an extra class rust,nightly. I am not sure what solution would fit better. :)

[steveklabnik]

Yeah, that idea would work as well. What's nice about the former approach is that it Just Works, but the latter version is nice because it gives you a bit more control.