Don't advertise hash sign usage with '-b' option

Most shells would interpret the hash sign as start of the comment, so command
with an unescaped hash sign will result in rather puzzling error message:

  $ imv -b #C0C0C0 files...
  Unknown argument 'b'. Aborting.

Although there is obvious workaround (detect 'b' in '?' case of getopt switch
and put more appropriate message), it generally makes more sense to suggest
plain hex values for input.

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