Fix code blocks with comma separated classes

[azerupi]

Fixes #155

This PR iterates over all code blocks and replace all commas in the class attribute by spaces. This allows to specify multiple classes in code blocks. This is mostly for the Rust book that uses:

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

@steveklabnik Is this better than #213 ?

[steveklabnik]

yes!

[steveklabnik]

https://doc.rust-lang.org/nightly/book/

🎊 🎊 🎊 🎊 🎊 🎊 🎊 🎊 🎊

Do you think we could get a release cut with this? https://doc.rust-lang.org/nightly/book/associated-types.html#trait-objects-with-associated-types is a thing now 😄

[azerupi]

Yes, sure :)
I will take this opportunity to test out cargo-release which should make it easier.

Also, I think it would be a good idea to add you to the crate maintainers on crates.io so that you can push releases too. I'm not sure how I should do that but I'm sure it is documented somewhere :)

Edit: Waoh! That was smooth 😮 I am impressed by cargo-release
Edit 2: Voila, you are now co-owner of mdBook on crates.io :)