Add configuration to enable mermaid

This ensures that the extra resources are only loaded if the author wishes to use mermaid diagrams.

Had to use reveal dependencies to ensure the first load works although the dependency mechanism is deprecated in later versions. Loading the dependencies with `async: true` didn’t work.

_config.yml

@@ -30,11 +30,13 @@ reveal_options:
 
 # Additional reveal.js dependencies
 reveal_dependencies:
-  - "{ src: 'js/mermaid/mermaid.js' },"
 
 # Reveal.js subdirectory
 reveal_path: reveal.js/
 
+# mermaid.js diagram integration
+mermaid_diagrams: true
+
 # Exclude directories and/or files from the conversion
 exclude:
   - Gemfile*

_layouts/reveal.html

@@ -20,8 +20,9 @@
 		<link rel="stylesheet" href="{{ css_file }}">
 	{% endfor %}
 {% endif %}
+{% if site.mermaid_diagrams %}
 		<link rel="stylesheet" href="{{ 'css/mermaid.css' | relative_url }}">
-
+{% endif %}
 		<!-- For syntax highlighting -->
 		{% if site.highlight_style_sheet %}
 			{% assign highlight_style_sheet = site.highlight_style_sheet %}
@@ -85,9 +86,6 @@
 			</div>
 
 		</div>
-
-		<script defer src="https://cdn.jsdelivr.net/npm/[email protected]/dist/mermaid.min.js"></script>
-
 		<script src="{{ site.reveal_path }}lib/js/head.min.js"></script>
 		<script src="{{ site.reveal_path }}js/reveal.js"></script>
 
@@ -123,6 +121,10 @@
 					{ src: '{{ site.reveal_path }}plugin/markdown/markdown.js', condition: function() { return !!document.querySelector( '[data-markdown]' ); } },
 					{ src: '{{ site.reveal_path }}plugin/highlight/highlight.js', async: true, callback: function() { hljs.initHighlightingOnLoad(); } },
 					{ src: '{{ site.reveal_path }}plugin/zoom-js/zoom.js', async: true, condition: function() { return !!document.body.classList; } },
+					{% if site.mermaid_diagrams %}
+					{ src: 'https://cdn.jsdelivr.net/npm/[email protected]/dist/mermaid.min.js'},
+					{ src: '{{ 'js/mermaid/mermaid.js' | relative_url }}'},
+					{% endif %}
 					{% if site.reveal_notes_server %}
 					{ src: '{{ site.reveal_path }}socket.io/socket.io.js', async: true },
 					{ src: '{{ site.reveal_path }}plugin/notes-server/client.js', async: true }