Keep last header and footer unless a new one is set

[bjornasm]

Description

I think working with quarto to make presentations is really enjoyable. Right now I use the revealJS format. However I have found myself in a pattern where I might repeat a heading over several slides (essentially the content under that heading spans several slides) - which means that in my markdown I also repeat the same heading over and over. This breaks up a lot of the natural flow of the markdown text.

One idea I had is that you could opt in to let headings (and possibly footers) be "sticky". By that I mean that unless you set a new header or footer you repeat the previous one. This would mean that a document could flow naturally:

## Feature requests

The first feature request that I talk about (...)

---

The second feature request that I talk about (..)

Right now this would be something like:

## Feature requests

The first feature request that I talk about (...)

---
## Feature requests

The second feature request that I talk about (..)

[mcanouil]

--- has already many meanings (horizontal rule, new slide, headings delimiter, YAML block delimiters), I don't think adding one is a good idea.

You can solve your main issue using a Lua filter to repeat whatever you want on the slide that use --- (surrounded by empty lines).