Provide the option to choose for 1 atomic update/delete operation.

As mentioned in issue #73 i've implemented an option which defaults to the current use, but allows you to choose for 2 step update or remove of documents, in order to allow mongoose to call the pre and post hooks on a schema.

It is up and running in our application, as mentioned in the issue, we use it for document versioning where new versions are saved after every save or remove call. 

I can add or refactor if you want.  Let me know what you think.