Interactive quizzes for Markdown

Examples of the quizzes have been added to The Rust Book.

Experiment: Improving the Rust Book: https://rust-book.cs.brown.edu/

A small but interesting detail: the quizzes open as a full-screen in the Rust book. The explanation:

> Why is this quiz fullscreen?

> We want to know how much you are learning that can be recalled without assistance. Please complete the quiz without re-reading the text.