minor readme changes

New option for specifying quality and unsharpen values. New regex and…

… filename extension checking on recurse

Added a sizes option for specify responsive image sizes. Colored term…

…inal output

Changed resized images to 480,640,768,960,1024,1366,1600 and 1920

added prefix option, metrics and elapsed time

Didactic changes in readme.

Added a quiet option to supress error warnings