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

incorporealcms/journal.py

@@ -20,4 +20,5 @@ def journal_file_resolver(path):
     """
     if path.endswith('/'):
         path = f'{path}index'
+    path = f'{path}.md'
     return path