New plugin Optimize Images

This plugin applies lossless compression on JPEG and PNG images, with no
effect on image quality. It uses [jpegtran][1] and [OptiPNG][2]. It assumes
that both of these tools are installed on system path.

[1]: http://jpegclub.org/jpegtran/              "jpegtran"
[2]: http://optipng.sourceforge.net/            "OptiPNG"