code: function severity2num (severity) {
  	if (!severity) {
  		severity = 0;
  	} else if (severity === 'error') {
  		severity = 1;
  	} else if (severity === 'warn') {
  		severity = 2;
  	} else if (severity === 'info') {
  		severity = 3;
  	} else {
  		severity = 4;
  	}
  	return severity;
  }
docs: 将错误等级的字符串转化为排序所用的权重
  @param  {String} severity 错误等级
  @return {Int}          排序权重