If you continue with John’s answer, if you want two adjacent numbers to have signatures under one signature, the “hack” would be to do this:
{width=60%} {width=40%} \begin{figure}[!h] \caption{A single caption for the two subfigures} \end{figure}
This results in a single title for two images placed side by side. You may need to adjust each individual image width parameter or header placement qualifier !h to make it look like this:

I found this useful because you do not need to download a picture from the Internet, as in the pure LaTeX \subfigure . Those. just use pandoc markdown to get the image, and LaTeX to generate the title.
If you want to go crazy, you can use the same idea as above to create sub-figure signatures, for example:
{width=60%} {width=40%} \begin{figure}[!h] \begin{subfigure}[t]{0.6\textwidth} \caption{Caption for the left subfigure} \end{subfigure} \hfill \begin{subfigure}[t]{0.4\textwidth} \caption{Caption for the right subfigure} \end{subfigure} \caption{A single caption for the two subfigures} \end{figure}

Edit 20180910:
You will need to include the following packages in the pandoc YAML header / header:
header-includes: | \usepackage{caption} \usepackage{subcaption}
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