The current implementation was wrong as it also percent encoded query strings
(which should be plus encoded) and calling urllib.quote on the path (and
urllib.quote_plus on the query string) assumes the url is not already encoded.
What if the document author pasted a url that was already encoded? She probably
did not intend for `%20` to become `%2520`. Or did she? It is now clear to me
why many implementation do nothing to urls. Just pass them though as-is. To bad
if they are not valid HTML. HTML authors have to encodee their own urls, so I
guess markdown authors have to as well.