Swap RST's use of `object` for SVG to use `image`

Closes https://github.com/github/markup/issues/264.

Similar to https://github.com/github/markup/pull/256, the trick here was to [look at the RST generation code](http://sourceforge.net/p/docutils/code/HEAD/tree/tags/docutils-0.11/docutils/writers/html4css1/__init__.py#l1019) and figure out how to override the method to do what we need it to do.

~~We're plucking attributes off the `object` and applying them to the `image`. These attributes might seem somewhat arbitrary at first:~~

~~* src~~
~~* alt~~
~~* width~~
~~* height~~

~~The original code also calls for `style`, `class`, `align`, etc., but we strip these out with sanitization anyway.~~ **Edit**: this is no longer true, see below.

/cc @bkeepers @github/user-content @github/prose Which team is best pinged for these sorts of issues?