Feature request: Collapsible menu?

[xiahualiu]

This is a great theme! I wonder if the chapters can be collapsed/hidden?

Like if I am reading chapter 2, I don't want to see the tabs in chapter 1 on the side navigation menu. Also if I am reading section 1 of chapter 2, I want none but only sub sections of section 1 shown on the side menu.

Make menu collapsible is great for reading big document which may have hundreds of sections and sub sections.

[Keats]

It shouldn't be too hard to implement (even with only CSS I think?) and make it available as an option if someone wants to try.