opticharger: Add pngquant

* pngquant is a command-line utility and a library for converting 24/32-bit PNG images to paletted (8-bit) PNGs.

The conversion reduces file sizes significantly (often as much as 70%) and preserves full alpha transparency. Generated images are compatible with all modern web browsers, and have better fallback in IE6 than 24-bit PNGs.

Read more info at http://pngquant.org/

From my tests the overall compression reduces final flashable zips by 7-10 MBs.  The reduction of quality in pngs is also still slight enough as to avoid detection.

Signed-off-by: Chester Kener <Cl3Kener@gmail.com>