Mutually exclusive groups
*************************

A group of options can be marked as mutually exclusive using
":mutex(option, ...)" method of the Parser class.

   parser:mutex(
      parser:flag "-q --quiet",
      parser:flag "-v --verbose"
   )

If more than one element of a mutually exclusive group is used, an
error is raised.

   $ lua script.lua -qv

   Usage: script.lua ([-q] | [-v]) [-h]

   Error: option '-v' can not be used together with option '-q'
