Allow conditional or alternative images in markdown

[gaborcsardi]

WRE suggests SVG + PDF for nice images in HTML and PDF:

\if{html}{\figure{Rlogo.svg}{options: width=100 alt="R logo"}}
\if{latex}{\figure{Rlogo.pdf}{options: width=0.5in}}

https://cran.r-project.org/doc/manuals/R-exts.html#Figures

As a first step we could support conditional images. My use case is adding SVG images to the HTML manual, and I don't mind if they are omitted from the PDF manual.

Ideally we would also support images created by code blocks, in addition to the ones put in "manually" with the markdown image syntax.

[gaborcsardi]

We could also select the output format automatically. E.g. If it is an SVG, it should be HTML only. If it is a PDF, it should be PDF only. GIF, JPEG, PNG are good for both. WRE says that these are the only types that are handled by default:

Files with extensions .jpg, .jpeg, .pdf, .png and .svg from that directory will be copied to the help/figures directory at install time.