Add scroll: view option to revealjs template

[memeplex]

Describe your proposed improvement and the problem it solves.

https://revealjs.com/scroll-view/

It provides a vertical document view for the entire slideshow.

Besides providing an alternative nice-looking navigation style, it may help with some previewers that reset the position of the slideshow after each refresh.

Describe alternatives you've considered.

Creating a custom template.

[memeplex]

A nice demo: https://revealjs.com/demo/?view=scroll

[jgm]

Seems reasonable. Want to make a PR?

[memeplex]

Yes, no problem, it will take me a few weeks though because I'm quite busy right now. It's just modifying the templates, isn't it?

[jgm]

Yes, I think that's all that is needed.