Add length-handling to xterm-colour lister
Add length-handling to xterm-colour lister

The utility now only lists 16 colours by default, with the full spectrum
accessed by passing either -a ("All") or -l ("Long"). Custom lengths can
can also be used by setting the -c ("Count") option.

    $ rainbow -a      # Generates a 256-colour list
    $ rainbow -l      # Same as above
    $ rainbow -c 128  # Generates a 128-colour list

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