Add 'selected' as toc item state

[jakauppila]

Description

It'd be great if there was a data-md-state of selected in the toc to allow for some styling of the currently selected item.

Basically, I want to replicate the styling available in AWS's documentation: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/concepts.html

[squidfunk]

Sure, feel free to PR. Otherwise, I may add it at some point in the future when I find some time.

[squidfunk]

Implemented in the (currently not building) refactor/rxjs-typescript branch:

This feature will be part of 5.0.0 which is separately tracked in #1306. With some JavaScript it will now also be possible to style the containing parent sections, so if a h3 is active, the h1 and h2 are now also tracked.

[squidfunk]

Material for MkDocs 5.0.0rc1 is out which fixes this issue 😀 Let's test and improve it together!