Add missing variable in templates regarding toc

[cderv]

This will partly bring support for #123 by updating the template and adds the same argument variable than in html_document()

We can deal with the default value to set in another PR

This means that this will now work

---
title: "Untitled"
output: 
  revealjs::revealjs_presentation: 
    toc: true
toc-title: outline
---

## Slide 1

Content

## Slide 2

Content

• A title to the toc will be added
• TOC slide can be styled using id #TOC selector
• TOC title can be styled using id #toc-title
• Templates is up to date with pandoc one on this.