Support unnumbered table/figure captions

[Omikhleia]

Opened further to discussion #34 (reply in thread)

With the resilient.book class, one can do in SIL-language:

\begin{figure}
(some figure with a numbered caption)
\caption{yyy}
\end{figure}

\begin[numbering=false]{figure}
(some figure with an unumbered caption)
\caption{yyy}
\end{figure}

In markdown, I would naively have expected the following to do the same thing:

![some figure with numbered caption](examples/images/someimage.png){ width=5cm }

![some figure with unumbered caption](examples/images/someimage.png){ .unnumbered width=5cm }

Apparently I forgot it and we do not propagate the necessary options ;)