misc/flag-icons: new port had been added (+)

A curated collection of all country flags in 4x3 and 1x1 formats
converted from SVG to PNG for various sizes, from 24px to 256px,
with rounded corners (based on the blog post by Joel Dare).

Depend on ImageMagick 6.x for the time being until I can come up
with a nice version-agnostic way to circumvent the WARNING: The
"convert" command is deprecated in IMv7, use "magick" instead of
"convert" or "magick convert".