replace PNG images with SVG

[frederic-mahe]

A possible enhancement: if plots could be served as SVG or any another vectorial format rather than PNG, primer names and other texts would be searchable, and plots would be scalable to any screen size.

[vaulot]

This will be a nice enhancement but I will keep for the next round. By default shiny exports as PNG and I would need to use another package for SVG:
https://davidgohel.github.io/ggiraph/articles/offcran/using_ggiraph.html