Adds optional 'quality' param to allow lossy jpeg output

Depending on formatting of the original image, slight lossy optimization can provide huge file-size savings over strictly lossless JPEG compression. This update adds an optional 'quality' param that can be sent along with the source image path.

Optimizer will default to lossless compression if no quality param is specified, this change will not break existing implementations of the optimizer. Updates jpegoptim command line calls to force Progressive JPG output, as they provide a better end-user experience when loading over a slow connection. PNG optimization untouched; PNGs will ignore the quality param. Readme updated to reflect new options and defaults.