don't error on breadcrumbs if a dir doesn't have index.md

fixes #8

tests/instance/pages/no-index-dir/page.md

@@ -0,0 +1 @@
+this is a test page

tests/test_pages.py

@@ -35,6 +35,16 @@ def test_generate_page_navs_subdir_with_title_parsing_real_page(app):
         ]
 
 
+def test_generate_page_navs_subdir_with_no_index(app):
+    """Test that breadcrumbs still generate even if a subdir doesn't have an index.md."""
+    with app.app_context():
+        assert generate_parent_navs('pages/no-index-dir/page.md') == [
+            ('incorporeal.org', '/'),
+            ('/no-index-dir/', '/no-index-dir/'),
+            ('page', '/no-index-dir/page')
+        ]
+
+
 def test_render_with_user_dark_theme(app):
     """Test that a request with the dark theme selected renders the dark theme."""
     cookie = dump_cookie("user-style", 'dark')