Allow superscript and subscript in sanitized HTML.

This is useful for READMEs and the like -- the tags `sup` and `sub` are
not harmful, and allow for things like footnotes.

Signed-off-by: Julia DeMille <me@jdemille.com>

srht/markdown.py

@@ -215,6 +215,7 @@ _sanitizer = bleach.sanitizer.Cleaner(
         "details", "summary",
         "abbr", "dfn",
         "del",
+        "sup", "sub"
     ],
     attributes={**bleach.sanitizer.ALLOWED_ATTRIBUTES, **_sanitizer_attrs},
     protocols=[