code: public <V, T extends Enum & Option> OptionsMapper env(final String name, final T option,
                                                            final Function<String, V> converter) {
          register("env: " + name, option, System.getenv(name), converter);
          return this;
      }
docs: Map environment variable value to option.
  
  @param name      environment variable name
  @param option    option
  @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