tp 字段排除功能 --查询数据库时排除字段查询剩余字段

举例：user表有id,name,age,status 4个字段

$model = M('User');
$model->field('*')->select();//SELECT * FROM user
$model->field(true)->select();//SELECT id,name,age,status FROM user
$model->field('status',true)->select();//SELECT id,name,age FROM user