Add --multistream option to comm?

[dspinellis]

• Without the option is sends all output to stdout, in order to preserve backward compatibility.
• Update examples
• Document in index.html

[dspinellis]

Alternatively, modify the API to allow the specification of the number of I/O channels that the command can accept, and adjust according to what's available upstream/downstream. But then comm | tee might do the wrong thing. Also, for generality we'd need --multistream-input and --multistream-output. Adding these options to tee, cat and other adapted tools would be inelegant.