Support rendering images in Markdown footnotes

[cvincent]

Example:

- My list
- Of things[^1]
- Etc

[^1]: ![image](path/to/image.png)

[cvincent]

The above example actually works, I had just perhaps misformatted my footnotes. The problem occurs when footnotes are separated by a newline:

[^1]: Some footnote

[^2]: ![image](path/to/image.png)

The above does not work, but it could actually be an issue with render-markdown.nvim, as it also does not render the footnote number in superscript (only when the footnote is an image; it renders superscript with newlines just fine otherwise).