Use stdin and stdout with 2ff
It always seemed odd to have a file-argument for 2ff, given all other farbfeld conversion tools operate on stdin and stdout only. Given I received numerous e-mails on this matter I've decided to change the usage to lessen the confusion. Maybe there's a way to do some magic with tee(1), but I didn't bother just yet. To ease the transition, the number of arguments are checked and a usage printed if arguments are given.
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