Single file Java

Reducing the complexity of having to keep a jar file and launch through the Java VM, this version can be run through the modern Java 11 single file support.

Download, make executable using "chmod +x PNGconv", move it to a suitable folder in your PATH. Run:

PNGconv [mode] [-option1] [-option2] [-option...]

First public release

PNGconv was written for my own personal use. After having gotten some input that others might benefit from it as well I added some suggested functionality and here we are. Pull requests appreciated.