build: create script to create many icon sizes from single input #14

Generating the many specific icon sizes is a pain. Would be nice to take a single icon image file of the maximum size, and generate all other sizes from it. Most of the libraries that do this, like this gulp helper, require ImageMagick to already be installed on the system. I wonder if there's an easier way without requiring the user to install an image library. Could even be a remote service.

See the table of icon sizes here for what should be provided for Safari (look at the "Web clip icon" row of the table).
https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html#//apple_ref/doc/uid/TP40006556-CH27

According to this article, 192x192, 144x144, and 96x96 are reasonable, but the algorithm looks for icons that are 48dp * display density.
https://developers.google.com/web/updates/2014/11/Support-for-installable-web-apps-with-webapp-manifest-in-chrome-38-for-Android?hl=en