Add custom class support for mermaid and button shortcodes

exampleSite/content/docs/shortcodes.md

@@ -34,7 +34,7 @@ Miseratus fonte Ditis conubia.
 # Buttons
 
 ```tpl
-{{</* button relref="/" */>}}Get Home{{</* /button */>}}
+{{</* button relref="/" [class="..."] */>}}Get Home{{</* /button */>}}
 {{</* button href="https://github.com/alex-shpak/hugo-book" */>}}Contribute{{</* /button */>}}
 ```
 
@@ -134,7 +134,7 @@ Miseratus fonte Ditis conubia.
 
 {{< columns >}}
 ```tpl
-{{</* mermaid */>}}
+{{</* mermaid [class="text-center"]*/>}}
 sequenceDiagram
     Alice->>Bob: Hello Bob, how are you?
     alt is sick

exampleSite/resources/_gen/assets/scss/book.scss_48b060fe05b0a273d182ef83c0605941.json

@@ -1 +1 @@
-{"Target":"book.min.40a7a4a197e0c73ece96b2afec0d34ad394563de6f97699dd68b042cd2f5ae9e.css","MediaType":"text/css","Data":{"Integrity":"sha256-QKekoZfgxz7OlrKv7A00rTlFY95vl2md1osELNL1rp4="}}
+{"Target":"book.min.1f05d81b275a862f44a849039ab95c032a03abf4aebe5aeff5367876fc7b68ab.css","MediaType":"text/css","Data":{"Integrity":"sha256-HwXYGydahi9EqEkDmrlcAyoDq/Suvlrv9TZ4dvx7aKs="}}