morphline protocol buffer command

Consider using an enum instead of string for the extractMethods in combination with the Validator util. Similar to the what the GrokBuilder does:

      this.numRequiredMatches = new Validator<NumRequiredMatches>().validateEnum(
          config,
          getConfigs().getString(config, "numRequiredMatches", NumRequiredMatches.atLeastOnce.toString()),
          NumRequiredMatches.class);