Implement multi-slide macros

[yeldiRium]

Hi!

Since I wanted to use the macros to stagger lists into slides and needed to generate multiple slides with macros as suggested in #488, I implemented the feature.

The behavior is now: When a macro is used inline and emits multiple slides, the first slide emitted is appended to the currently active slide, which is then pushed. The following slides are all pushed except the last one, which becomes the new active slide on the stack.

My reasoning for this behavior is the following scenario:

...
---

![:stagger](
# headline

%%%
- list element
  - sub element
  - sub element
%%%
- list element
%%%
- list element
%%%
- list element
%%%
- list element
%%%
- list element
)

---
...

With this macro implementation:

remark.macros.stagger = function () {
  return this.split('%%%\n').reduce(({ output, current }, part) => {
    const next = current + part
    return {
      output: output + next + '---\n',
      current: next
    }
  }, { output: '', current: '' }).output;
}

Looking forward to any feedback and with kind regards, yeldiR

[utdrmac]

What does this do that the standard way does not?

---
## header of slide

* topic 1
--
  * sub topic 1a
--
  * sub topic 1b
--
* topic 2
--
* topic 3

[yeldiRium]

Your example creates multiple slides with one part of the list each, right? Or am I missing a feature?

I want to create multiple pages with an incrementally larger part of the list showing. As I see it that is currently not possible with or without macros.

[utdrmac]

Here's what I get with the standard stuff. This looks to me like "multiple pages with an incrementally larger part of the list"

incremental.pdf incremental-bullet.html

[yeldiRium]

Huh. I wasn't aware of that. In that case, no macro is needed.

Thank you!

[yeldiRium]

The multi-page macro feature could of course be useful regardless, so if anyone needs it, feel free to reuse this pull request.