adding supporte for draft chapter

[Tudyx]

mdbook allows you to define draft chapter in your summary.

But currently if you have a draft chapter and we use mdbook-quiz we panic because of this line https://github.com/willcrichton/mdbook-quiz/blob/main/src/processor.rs#L207

As a minimal reproduction case, we can simply use the example provided in this repository and add this line to SUMMARY.md :

# Summary

- [Chapter 1](./chapter_1.md)
- [Chapter 2]()

I think the simple fix would be doing only treat the case were the option is Some using a if let Some construct.

[willcrichton]

Closed by #10.