Add 'active' class to current page in rustbook table of contents

[cllns]

Currently the table of contents for rustbook doesn't signify which page you are on.

This PR adds an 'active' class to the link for the current page, and defines the CSS rule for that class to make the link underlined and bold.

Not sure about two things:

1. Is current_page is a good name for the function parameter? At first I thought current_item would be good, but then in the walk_item function, you'd have item and current_item.

2. For the CSS, is both bold and underline too much? At first I had it just be underlined, but that's also how the links look when they're hovered over.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[killercup]

Nice! I've been meaning to add something like this as well. I'm no authority, but current_page sounds good (maybe current_chapter?). I'd also just set it to bold, without adding an underline.

[alexcrichton]

Thanks! Would it be possible to upload a preview of these docs as well?

[cllns]

Here you go @alexcrichton: http://cllns.com/rustbook/ (this link will not work once this PR is closed)

@killercup thanks for the suggestion! If someone else also prefers to not have the current page underlined, I'll change it :)

[steveklabnik]

I like it 👍

[alexcrichton]

@bors: r+ 99868f6 rollup

Awesome, thanks!