Implemented --exec option.

--exec is a paramter that allows the user to execute a shell command after a file is finished downloading and being modified by the other post-processors. It is based upon find's -exec syntax where any occurrence of {} will be replaced with the file name in single quotes.
Why is this class not inheriting from `PostProcessor`? Also, as a convention, all post processor class names should end in `PostProcessor` or `PP`.