graphviz: allow overriding the engine

This can be done like so:

  ```graphviz {.engine="neato"}
  digraph diagram {
    A -> B
  }
  ```

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

Author: dscho

layouts/_default/baseof.html

@@ -25,6 +25,7 @@
           graphAttributes: {
             bgcolor: "transparent",
           },
+          engine: x.getAttribute("engine") || "dot",
         }
         const svg = vizInstance.renderString(x.innerText, options)
         const img = document.createElement('img')

layouts/_markup/render-codeblock-graphviz.html

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