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/functional_markdown_tests.py

@@ -0,0 +1,29 @@
+"""Test graphviz functionality."""
+import os
+
+from incorporealcms import create_app
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+
+
+def app_with_pydot():
+    """Create the test app, including the pydot extension."""
+    return create_app(instance_path=os.path.join(HERE, 'instance'),
+                      test_config={'MARKDOWN_EXTENSIONS': ['incorporealcms.mdx.pydot']})
+
+
+def test_functional_initialization():
+    """Test initialization with the graphviz config."""
+    app = app_with_pydot()
+    assert app is not None
+
+
+def test_graphviz_is_rendered():
+    """Initialize the app with the graphviz extension and ensure it does something."""
+    app = app_with_pydot()
+    client = app.test_client()
+
+    response = client.get('/test-graphviz')
+    assert response.status_code == 200
+    assert b'~~~pydot' not in response.data
+    assert b'data:image/png;base64' in response.data

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