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.
As a rule of thumb, regular expression strings should be raw strings, i.e. prefixed with an `r`. This makes good editors apply highlighting and prevents a number of errors.