refactor: move Reveal.JS plugin to its own module

```
module:
  imports:
    - path: github.com/wowchemy/wowchemy-hugo-themes/modules/wowchemy-plugin-reveal
```

See #2792

modules/wowchemy-plugin-reveal/README.md

@@ -0,0 +1,22 @@
+# wowchemy-plugin-reveal
+
+A Wowchemy/Hugo plugin for [Reveal.JS](https://revealjs.com/).
+
+Empowers you to easily create, share, and present slide decks using the open, future-proof Markdown standard.
+
+Supports math, syntax highlighting, diagrams, speaker notes, and much more!
+
+## Install
+
+1. Add the module to your `config/_default/config.yaml` alongside the core Wowchemy module:
+
+   ```yaml
+   module:
+     imports:
+       - path: github.com/wowchemy/wowchemy-hugo-themes/modules/wowchemy
+       - path: github.com/wowchemy/wowchemy-hugo-themes/modules/wowchemy-plugin-reveal
+   ```
+
+## Usage
+
+[View the documentation](https://wowchemy.com/docs/content/slides/)

modules/wowchemy-plugin-reveal/assets/js/wowchemy-slides.js

@@ -1,3 +1,10 @@
+/*************************************************
+ *  Wowchemy - https://wowchemy.com/
+ *  License: https://github.com/wowchemy/wowchemy-hugo-themes/blob/main/LICENSE.md
+ *
+ *  Reveal.JS integration
+ **************************************************/
+
 /*
   global RevealMarkdown, RevealSearch, RevealNotes, RevealMath, RevealZoom, Reveal, mermaid, RevealMenu
 */

modules/wowchemy-plugin-reveal/config.yaml

@@ -0,0 +1,6 @@
+module:
+  mounts:
+    - source: assets
+      target: assets
+    - source: layouts
+      target: layouts

modules/wowchemy-plugin-reveal/go.mod

@@ -0,0 +1,3 @@
+module github.com/wowchemy/wowchemy-hugo-themes/modules/wowchemy-plugin-reveal
+
+go 1.15

modules/wowchemy-plugin-reveal/layouts/slides/baseof.html

@@ -4,7 +4,6 @@
 
   {{ $cdn_url_reveal := "https://cdn.jsdelivr.net/npm/reveal.js@4.3.1" }}
   {{ $js := site.Data.assets.js }}
-  {{ $css := site.Data.assets.css }}
 
   <meta charset="utf-8">
   <meta name="viewport" content="width=device-width, initial-scale=1">
@@ -38,9 +37,12 @@
   {{ end }}
 
   {{/* Load any Reveal CSS customizations */}}
-  {{ $css_custom := resources.Get "css/reveal_custom.css" }}
-  {{ $style := slice $css_custom | resources.Concat "css/reveal_custom.css" | resources.Minify }}
-  <link rel="stylesheet" href="{{ $style.RelPermalink }}">
+  {{ $custom_reveal_css := printf "css/reveal_custom.css" }}
+  {{ if (fileExists (printf "assets/%s" $custom_reveal_css)) }}
+    {{ $css_custom := resources.Get "css/reveal_custom.css" }}
+    {{ $style := slice $css_custom | resources.Concat "css/reveal_custom.css" | resources.Minify }}
+    <link rel="stylesheet" href="{{ $style.RelPermalink }}">
+  {{ end }}
 </head>
 <body>
 

scripts/view-starter-dev.sh

@@ -3,6 +3,7 @@
 HUGOxPARAMSxCMSxLOCAL_BACKEND=true \
 HUGO_MODULE_REPLACEMENTS="github.com/wowchemy/wowchemy-hugo-themes/modules/wowchemy/v5 -> ../../../modules/wowchemy,
 github.com/wowchemy/wowchemy-hugo-themes/modules/wowchemy-plugin-netlify -> ../../../modules/wowchemy-plugin-netlify,
+github.com/wowchemy/wowchemy-hugo-themes/modules/wowchemy-plugin-reveal -> ../../../modules/wowchemy-plugin-reveal,
 github.com/wowchemy/wowchemy-hugo-themes/modules/wowchemy-plugin-netlify-cms -> ../../../modules/wowchemy-plugin-netlify-cms" \
-hugo server --panicOnWarning --renderStaticToDisk -F --port 80 --bind 0.0.0.0 \
+hugo server --panicOnWarning --renderStaticToDisk -F --port 8081 --bind 0.0.0.0 \
 --source "starters/$1"