Multiple classes for code blocks

[azerupi]

The rust book uses annotations for rustdoc to test code snippets, like the following:

```rust,should_panic
fn main() {
    // Code here
}
```

But when rendering, the language class ends-up being language-rust,should_panic instead of just language-rust. Stripping the part after the comma or adding spaces would help.

See rust-lang/book#96