Support default fields and custom fields

[jiangz222]

• Default fields:
  Fields like CreateAt and UpdateAt, user doesn't need to operate those fileds when do insert or update operation
• Custom fields:
  User can define their custom fields about create and update time, like Create_At, Update_At with different type like time.Time or int64

[jiangz222]

• Default filed was partially implement in Hooks #71 , fields CreateAt and UpdateAt can update as default fields by insert field.DefaultField in document struct:

field.DefaultField `bson:",inline"`

• Custom field was partially implement in Hooks #71 too, user can define their custom fields about create and update time, with type time.Time or int64. You can check examples here:

func (u *UserHook) CustomFields() field.CustomFieldsBuilder {
	return field.NewCustom().SetCreateAt("CreateTimeAt").SetUpdateAt("UpdateTimeAt")
}

• But, function about default or custom fields only work in insert operation (InsertOne/InsertMany). 😢
  More job need to do in update operation.

[jiangz222]

#78 support this issue