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.
Again, no reason to use re in this form. Also, this allows extractors to execute arbitrary commands. Make sure to include a test case where the file name contains quotes.