Added support for reading settings from a config file
If a config file is passed on the command line, we will read the settings from it rather than show the ui.
Changed how the CSS file is generated.
Rather than generate 2 css classes that must both be used we now generate a single css class per image.  The CSS class
name now includes the sprite name as well as the individual image name plus an optional prefix.  This should make it
easier to work with a bunch of sprite images at once.