Add custom class support for mermaid and button shortcodes

layouts/shortcodes/button.html

@@ -7,4 +7,6 @@
 {{ with .Get "relref" }}
   {{ $ref = relref $ . }}
 {{ end }}
-<a {{ with $ref }} href="{{.}}" {{ end }} {{ with $target }} target="{{.}}" {{ end }} class="book-btn">{{ $.Inner }}</a>
+<a {{ with $ref }} href="{{.}}" {{ end }} {{ with $target }} target="{{.}}" {{ end }} class="book-btn{{ with .Get "class" }} {{ . }}{{ end }}">
+  {{ $.Inner }}
+</a>

layouts/shortcodes/mermaid.html

@@ -4,6 +4,6 @@
 {{ .Page.Scratch.Set "mermaid" true }}
 {{ end }}
 
-<p class="mermaid">
+<p class="mermaid{{ with .Get "class" }} {{ . }}{{ end }}">
   {{ .Inner }}
 </p>