Add ImageMagick C++ dependency

[moldach]

There are some underlying functions in the Magic C++ library that have not yet been added to the R-wrapper {magick} yet: ropensci/magick#316

Therefore it would be useful if this was added to shinyapps.io for those wishing to call functions from the command line with system() function, e.g.: magick convert images/qrcode.png -alpha set -background none -channel A -evaluate multiply 0.420 +channel images/qrcode.png