Describe the feature
gorm 关联删除目前没有物理删除的选项，存在deleted_at字段的时候，生成的sql是update
Motivation
删除user 并且删除user关联的account 目前写法：
db.Debug().Select("Account").Unscoped().Delete(&user)
执行结果是只物理删除了user表的内容，关联的account里面有deleted_at字段，是update操作。
想增加一个可以选择物理删除关联内容的功能
Related Issues
Comment From: lyn444
This has been done for HasOne and HasMany but not for Many2Many.
could we also add this to case schema.Many2Many
Comment From: lyn444
@jinzhu