Add support for speaker notes

This implements a system for speaker notes via `details` elements and
some JavaScript. The general idea is

1. You add speaker notes to each page by wrapping some Markdown code
   in `<details> … </details>`. This is a standard HTML element for,
   well extra details. Browsers will render the element with a toggle
   control for showing/hiding the content.

2. We inject JavaScript on every page which finds these speaker note
   elements. They’re styled slightly and we keep their open/closed
   state in a browser local storage. This ensures that you can keep
   them open/closed across page loads.

3. We add a link to the speaker notes which will open in a new tab.
   The URL is amended with `#speaker-notes-open`, which we detect in
   the new tab: we hide the other content in this case.
   Simultaneously, we hide the speaker notes in the original window.

4. When navigating to a new page, we signal this to the other window.
   We then navigate to the same page. The logic above kicks in and
   hides the right part of the content. This lets the users page
   through the course using either the regular window or the speaker
   notes — the result is the same and both windows stay in sync.

Tested in both Chrome and Firefox. When using a popup speaker note
window, the content loads more smoothly in Chrome, but it still works
fine in Firefox.

Fixes #53.

src/hello-world/small-example.md

@@ -18,3 +18,27 @@ fn main() {              // Program entry point
 }
 ```
 
+<details>
+
+The code implements the Collatz conjecture: it is believed that the loop will
+always end, but this is not yet proved. Edit the code and play with different
+inputs.
+
+Key points:
+
+* Explain that all variables are statically typed. Try removing `i32` to trigger
+  type inference. Try with `i8` instead and trigger a runtime integer overflow.
+
+* Change `let mut x` to `let x`, discuss the compiler error.
+
+* Show how `print!` gives a compilation error if the arguments don't match the
+  format string.
+
+* Show how you need to use `{}` as a placeholder if you want to print an
+  expression which is more complex than just a single variable.
+
+* Show the students the standard library, show them how to search for `std::fmt`
+  which has the rules of the formatting mini-language. It's important that the
+  students become familiar with searching in the standard library.
+
+</details>