clean up the default markdown configuration a bit

this brings the config a bit closer to my default, and updates the
expectations of a test accordingly

tests/functional_tests.py

@@ -7,7 +7,7 @@ def test_page_that_exists(client):
     """Test that the app can serve a basic file at the index."""
     response = client.get('/')
     assert response.status_code == 200
-    assert b'<h1>test index</h1>' in response.data
+    assert b'<h1 id="test-index">test index</h1>' in response.data
 
 
 def test_direct_file_that_exists(client):