In image tags, put the `src` attribute first.

This one might be more controversial, so I'm happy to discuss it.

MDTest1.1 wants all `src` attributes to be first. This is obviously not possible in the JsonML representation because Javascript objects have no order to their key/values. However, we can easily check if a src attribute is present and output it first in the rendering phase.

At first I thought this was silly and not worth doing, but after speaking to my team I realized that if we want HTML that reads nicely, the `src` attribute of an image should be first. You can debate secondary attributes, but for good looking HTML, the src is the most important and should be read first.