support array in chalkboard reveal_options

[atusy]

This PR fixes #62 by transforming character vector into an array represented by a bracketed string.

[atusy]

Oops, this is not enough. In some case, we need brackets even if length 1.

[atusy]

I improved handling of array requirements based on my reviewing JavaScript codes from chalkboard and menu.

FYI: when chalkboard theme is white, only first value of the color is available according to

revealjs/inst/reveal.js-3.3.0.1/plugin/chalkboard/chalkboard.js

Line 46 in 6b272a7

draw = [ drawWithPen , drawWithPen ];

[cderv]

Thanks @atusy !

I tweaked a bit to stay focus on the fix.

I think I'll rethink the whole handling of reveal options.

There is now a quoting thing to handle and the current fix I have made is maybe not the right solution.

Anyway, this correct fix #62

[cderv]

Tested with

---
title: "Test"
output: 
  revealjs::revealjs_presentation:
    self_contained: false
    reveal_plugins: ["chalkboard"]
    reveal_options:
      chalkboard:
        theme: whiteboard
        color: ['rgba(255, 0, 0, 0.3)', 'rgba(255, 0, 0, 0.3)']
    keep_md: true
---

# Slide 1

Test

* test
* test
* test

# Slide 2

more text

## subslide 1

even more text

[atusy]

Great! Thank you!!