provide markdown extension to render graphviz

this is server side, and a more standard format, and thus I like it more
than mermaid, which I've been using at work. but, I really wanted a
server-side option (see my manifesto) for drawing relationship graphs,
for D&D stuff of all things.

this adds an optional 'graphviz' feature to package installation which
consequently depends on pydot

tests/instance/pages/test-graphviz.md

@@ -0,0 +1,12 @@
+# test
+
+test
+~~~pydot:attack-plan
+digraph G {
+    rankdir=LR
+    Earth
+    Mars
+    Earth -> Mars
+}
+~~~
+more test