def fetch(flag)
      o = option(flag)
      if o.nil?
        cleaned_key = clean_key(flag)
        raise UnknownOption.new("option not found: '#{cleaned_key}'", "#{cleaned_key}")
      else
        o.value
      end
    end
def fetch(flag)
      o = option(flag)
      if o.nil?
        cleaned_key = clean_key(flag)
        raise UnknownOption.new("option not found: '#{cleaned_key}'", "#{cleaned_key}")
      else
        o.value
      end
    end
Returns an option's value, raises UnknownOption if the option does not exist.