The reasoning is that the translation of Markdown into HTML should be as
concise as possible. The semantics of the figure element seem to fit the
embedded nature of OEmbed content moreso than the div element which is
quite generic.
To keep a constant aspect ratio for the resulting iframe of an oEmbed
request in a responsive website, the iframe needs a div wrapper. With
this change, html results are wrapped in a "oembed" class div.
