Add support for Table of Contents / Chapters

[jacksonh]

Example article: https://timozander.de/blog/why-you-should-teach/

It looks like something like this query selector would get all heading text nodes: h1[data-omnivore-index], h2[data-omnivore-index], ....

[jacksonh]

Another example here: https://jacksonharper.substack.com/p/creating-a-test-post-with-sections

[DannieBGoode]

This is offered by Readwise already and it's very helpful. I agree with @jacksonh , it shouldn't be too complex to implement