add attr_list to markdown extensions

this will lead to me putting less HTML in the .md files, which is a good
thing

incorporealcms/config.py

@@ -35,6 +35,7 @@ class Config(object):
     MARKDOWN_EXTENSIONS = ['extra', 'mdx_linkify', 'smarty', 'tables']
     MARKDOWN_EXTENSION_CONFIGS = {
         'extra': {
+            'attr_list': {},
             'footnotes': {
                 'UNIQUE_IDS': True,
             },