Graphviz: Allow specifying layout engine on cmdline

It's better for demonstration purposes.

snippets/test_diff.py

@@ -37,7 +37,7 @@ def visualize_graphviz(diff):
 	graph.write('}\n')
 	
 	# Show:
-	gv = subprocess.Popen(['dot', '-Tgtk', '-Kfdp'], text=True, stdin=subprocess.PIPE)
+	gv = subprocess.Popen(['dot', '-Tgtk', '-K'+(sys.argv[2] if len(sys.argv)>=3 else 'fdp')], text=True, stdin=subprocess.PIPE)
 	gv.communicate(graph.getvalue())
 	gv.wait()