[FR] Enable revealjs code formatting options

[apreshill]

See: https://revealjs.com/code/

Specifically:

• [ ] Line numbers and highlights (https://revealjs.com/code/#line-numbers-%26-highlights)

```{r code-chunk, line.numbers = "3,8-10"}

- [ ] Step-by-step highlights (https://revealjs.com/code/#step-by-step-highlights)

Notes on discussion re: implementation:
add option hook with knitr, replace dot with dash for users (users will use the dot like `highlight.output` with xaringan), concatenated with `attr.source` in case users pile that on

[cderv]

I agree that reveal.js features are really nice.

On the same topic, currently the numberLined feature from Pandoc is not working ok.

---
title: "Reveal Template"
output:
  revealjs::revealjs_presentation: 
    keep_md: true
---

# Slide 1

```{r class.source = "numberLines", collapse = TRUE}
1 + 1
2 + 2
3 + 3

Probably something related to CSS with our package as it is working correctly with Pandoc default directly.