Markdown: add <small> to HTML whitelist

<small> is a better element for doing "side comments" instead of having
people hijack <sup> for that purpose. It will have to be typed in
manually since there's no markdown syntax for it, but I think it's good
to allow.

tildes/tildes/lib/markdown.py

@@ -77,6 +77,7 @@ HTML_TAG_WHITELIST = (
     "ol",
     "p",
     "pre",
+    "small",
     "strong",
     "sub",
     "sup",