graphviz: allow overriding the engine

This can be done like so:

  {{< graphviz engine="neato" >}}

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

assets/js/application.js

@@ -792,6 +792,7 @@ var Graphviz = {
         graphAttributes: {
           bgcolor: "transparent",
         },
+        engine: x.getAttribute("engine") || "dot",
       }
       const svg = vizInstance.renderString(x.innerText, options)
       const img = document.createElement('img')

layouts/shortcodes/graphviz.html

@@ -1,4 +1,4 @@
-<pre class="graphviz">
+<pre class="graphviz"{{ if (ne "" (.Get "engine")) }} engine="{{(.Get "engine")}}"{{ end }}>
   {{ .Inner | htmlEscape | safeHTML }}
 </pre>
 {{ .Page.Store.Set "hasGraphviz" true }}