Add new shortcode for spoiler blocks

Changed to using the details and summary tags. These are more POSH and
more accessibility friendly.

layouts/shortcodes/inDetails.html

@@ -0,0 +1,6 @@
+<div>
+    <details>
+        <summary>{{ .Get "title" }}</summary>
+        {{ .Inner | markdownify -}}
+    </details>
+</div>