add .md in journal_file_resolver

I'm confident enough that this is what I want to do for resolving
content, at least at the moment

tests/test_journal.py

@@ -3,12 +3,12 @@ from incorporealcms.journal import journal_file_resolver
 
 
 def test_journal_file_resolver_dir_to_index():
-    assert journal_file_resolver('/foo/') == '/foo/index'
+    assert journal_file_resolver('/foo/') == '/foo/index.md'
 
 
 def test_journal_file_resolver_subdir_to_index():
-    assert journal_file_resolver('/foo/bar/') == '/foo/bar/index'
+    assert journal_file_resolver('/foo/bar/') == '/foo/bar/index.md'
 
 
 def test_journal_file_resolver_other_requests_fine():
-    assert journal_file_resolver('/foo/baz') == '/foo/baz'
+    assert journal_file_resolver('/foo/baz') == '/foo/baz.md'