Enhance support for postprocessing

[mosbth]

The postprocessing option is a bit hardcoded right now, but there are several available tools for postprocessing tools.

This setting in the config file, and its usage in CImage, should be more generic with just an array for each image type, where one can specify any tool with any option and Cimage will walk through the list and postprocess it.

When doing this I should add test case for using Guetzli with JEPG.