Add custom class support for mermaid and button shortcodes

assets/_markdown.scss

@@ -28,6 +28,10 @@ $block-border-radius: 0.15rem;
     }
   }
 
+  img {
+    max-width: 100%;
+  }
+
   code {
     padding: 0 $padding-4;
     background: $gray-100;

assets/_utils.scss

@@ -34,6 +34,10 @@
   margin: 0 auto;
 }
 
+.text-center {
+  text-align: center;
+}
+
 @mixin fixed {
   position: fixed;
   top: 0;