layouts: add Graphviz support

Similar to TikZ support.

content/posts/2020-07-14-hello-world/index.md

@@ -8,6 +8,7 @@ tags:
 categories:
 favorite: false
 tikz: true
+graphviz: true
 ---
 
 ## Test post please ignore
@@ -40,6 +41,16 @@ echo hello world | cut -d' ' -f 1
   \end{tikzpicture}
 {{% /tikz %}}
 
+### Graphviz support
+
+{{% graphviz %}}
+  graph {
+    a -- b
+    b -- c
+    c -- a
+  }
+{{% /graphviz %}}
+
 ### Spoilers
 
 {{% spoiler "Don't open me" %}}