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Normalize IO streams, allowing for redirection of
+input+ and/or +output+, for example:

  $ foo              # => read from terminal I/O
  $ foo in           # => read from 'in' file, output to terminal output stream
  $ foo in out       # => read from 'in' file, output to 'out' file
  $ foo < in > out   # => equivalent to above (essentially)

Optionally a +block+ may be supplied, in which case
IO will be reset once the block has executed.

=== Examples

  command :foo do |c|
    c.syntax = 'foo [input] [output]'
    c.when_called do |args, options|
      # or io(args.shift, args.shift)
      io *args
      str = $stdin.gets
      puts 'input was: ' + str.inspect
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