code: public <V, T extends Enum & Option> OptionsMapper prop(final String name, final T option,
                                                             final Function<String, V> converter) {
          mappedProps.add(name);
          register(PROP_PREFIX + name, option, System.getProperty(name), converter);
          return this;
      }
docs: Directly map system property value to option.
  <p>
  Directly mapped properties excluded from mass processing in {@link #props(String)}, but direct mapping
  must appear before mass-processing.
  
  @param name      property name
  @param option    option
  @param converter value converter (may be null to use default converters)
  @param <T>       helper option type
  @param <V>       value type
  @return mapper instance for chained calls