code: public <V, T extends Enum & Option> OptionsMapper string(final T option, final String value,
                                                               final Function<String, V> converter) {
          register("", option, value, converter);
          return this;
      }
docs: Map string to option. When value is null or empty - nothing happens.
  
  @param option    option
  @param value     value string (could be null)
  @param converter value converter (may be null to use default converters)
  @param <V>       helper value type
  @param <T>       helper option type
  @return mapper instance for chained calls