Add institute variable in template for title slide

[cderv]

Proposal for #87

This only add the same variable than in pandoc template: https://github.com/jgm/pandoc-templates/blob/466c90ed5bb489d9cafe41b59fff47a3c5eb858c/default.revealjs#L55-L57

Title slide would look like this with our format currently

Example:

---
title: "Title slide"
author: Christophe
institute: RStudio
date: '`r format(Sys.Date(), "%D")`'
output: revealjs::revealjs_presentation
---

With Pandoc's current default using Quarto 0.2.144

---
title: "Title slide"
author: Christophe
institute: RStudio
date: '`r format(Sys.Date(), "%D")`'
format: revealjs
---

# Important {data-background="background.jpg"}

This is the first slide.

Using multiple authors and institues , in both cases it will only pile up the authors, then the institute. Do we want something clever than that for the title slide or be close to Pandoc's default ?

title: "Title slide"
author: 
  - Bob
  - Joe
institute: 
  - Great, PBC
  - Good, PBC
date: '`r format(Sys.Date(), "%D")`'
output: revealjs::revealjs_presentation

Opening this PR for discussion about this.

cc @apreshill

[cderv]

We'll stay simple for now and just to as the pandoc template is doing. This means that to provide several author, one would need to provide them in one string

---
title: "Untitled"
author: Bob, Joe
institute: One PBC, Two PBC
date: "09/08/2021"
output: 
  revealjs::revealjs_presentation: default
---

or use only the author field

---
title: "Untitled"
author: 
  - Bob, One PBC
  - Joe, Two PBC
date: "09/08/2021"
output: 
  revealjs::revealjs_presentation: default
---

So merging to just bring the documented pandoc variable https://pandoc.org/MANUAL.html#variables-for-html-slides