def key
      key = config[:key] || flags.last.sub(/\A--?/, '')
      key = key.tr '-', '_' if underscore_flags?
      key.to_sym
    end
def key
      key = config[:key] || flags.last.sub(/\A--?/, '')
      key = key.tr '-', '_' if underscore_flags?
      key.to_sym
    end
Returns the last key as a symbol. Used in Options.to_hash.