pydot: handle multiple dot images in one file

tests/instance/pages/test-two-graphviz.md

@@ -0,0 +1,25 @@
+# test
+
+test
+
+~~~pydot:attack-plan
+digraph G {
+    rankdir=LR
+    Earth
+    Mars
+    Earth -> Mars
+}
+~~~
+
+more test
+
+~~~pydot:new-attack-plan
+digraph H {
+    rankdir=LR
+    Venus
+    Mars
+    Venus -> Mars
+}
+~~~
+
+done