Support Incremental list items

[airtonix]

Is your feature request related to a problem? Please describe.

Currently the only increment supported at the moment, is revealing next slide.

Instead lets also provide incremental reveal of items within a slide.

Describe the solution you'd like

## Slide One

Welcome!

---

## Slide Two

What are incremental slides?

+ Incremental list items are defined as with `+`
+ Pressing `next slide` key will begin revealing them.
+ When there no more items to reveal `next slide` moves to the next slide.

Describe alternatives you've considered

Create clones of a slide where each slide progressivel contains more content.

## Slide One

Welcome!

---

## Slide Two

What are incremental slides? 

---

## Slide Two

What are incremental slides?

- Incremental list items are defined as with `+`

---

## Slide Two

What are incremental slides?

- Incremental list items are defined as with `+`
- Pressing `next slide` key will begin revealing them.

---

## Slide Two

What are incremental slides?

- Incremental list items are defined as with `+`
- Pressing `next slide` key will begin revealing them.
- When there no more items to reveal `next slide` moves to the next slide.

Additional context

This feauture exists in RevealJs.

[ariary]

Workaround

~> use script and preprocessing:
The script slide_incr:

echo "∙ First point"
read -p "" -n1 -s
echo "∙ Second point"

And put in your slide page:

~~~sh ./slide_incr
echo
~~~

[maaslalani]

Possible solution to this: #276 (comment)