Use atomic save instead of calling callbacks

Hi,

(Thanks for taking care of this great feature for mongoid)

Considering that we only update the field _keywords, I think it could be interesting to not trigger callbacks. To be more specific, I have issues with the rake task that use update_attribute because it saves every objects searchable.

If you think that an option should be provided instead of forcing the atomic save, I can code it.

Thanks again for this repo !