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.
Are you putting your code under [Unlicense](http://unlicense.org/)? If yes, then there shouldn't be an entry here (the commit history will be enough). If no, we can unfortunately not accept it.