Add themes in-tree

themes/github.com/davegallant/hugo-theme-gruvbox/layouts/_default/single.html

@@ -0,0 +1,20 @@
+{{ define "main" }}
+  <article class="post">
+    <div class="post-header">
+      <h1>{{ .Title | markdownify }}</h1>
+      {{ partial "post-meta.html" (dict "page" . "pageIndex" 0) }}
+    </div>
+
+    <div class="post-content">
+      {{ .Content }}
+    </div>
+
+    {{ if .Params.SocialShare }}
+      {{ partial "social-share.html" . }}
+    {{ end }}
+
+    {{ if .Params.Comments }}
+      {{ partial "comments.html" . }}
+    {{ end }}
+  </article>
+{{ end }}