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

[gjtorikian]

Closes #264.

Similar to #256, the trick here was to look at the RST generation code 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?

[ymendel]

Are we showing SVGs at all? I thought we blocked those for security reasons.

[gjtorikian]

@ymendel Oh we most certainly are showing them: https://github.com/gjtorikian/testing/blob/master/images.rst

Travis and other services provide the SVG option alongside PNG. We're just blocking the object tag for SVG, not img.

N.B. In 7a9da18 I just let all the attributes get applied to img. The sanitizer can sort out what to do with them.

[ymendel]

@gjtorikian: Ah, I've not kept up with the conversation, apparently. 🆒

I think applying all the attributes and letting the sanitizer deal with them is probably the right way to go.

[bkeepers]

👍 👍 Looks great.