Alt tags for other supported image types

[bmcfee]

PR #1112 implemented alt tag support for PNG and JPEG outputs (eg matplotlib figures) which can be set through cell tags.
It seems like this could be readily extended to other formats supported by IPython.display.set_matplotlib_formats, notably svg. (PDF might be a weird corner case for this, so maybe it's okay to ignore that.)

SVG support could be helpful for accessibility, since they would support magnification better than a raster format. It'd be a shame to have to use raster images just to get alt text.

[MSeal]

I'm all for it if someone wants to take a crack at adding the functionality. I can help review or point towards where specific code lives, but I don't have time currently to implement it myself.

Agreed that PDF might be a difficult case to deal, and might be ok to ignore.