[FeatureRequest] Adding Hooks/Signals for post/pre insert/delete/update operations?
I have a webapplication with a Django backend that I am currently rewriting in nim using prologue and norm. 

Django has a pretty neat signal system that allows you to run defined code before or after a data-manipulation-action such as insert, delete or update happens. In fact, I am implementing such a thing at the moment in my own side-project, because I made quite some use of it in my previous backend which was written in Django. In my case though, they are more "hooks" that you can use, rather than a proper signalling system.

The key idea of the implementation would be, to have pre-defined proc signatures that the user of norm *can* define and if they do define them, they are run. If they don't define them, that works just as well. For an example, I shall [refer to my sideproject](https://github.com/PhilippMDoerner/NimStoryFont/blob/master/src/applications/base_generics/genericArticleRepository.nim) (that is my abstraction layer between my application and norm. See the procs `createEntry`, `deleteEntry` and `updateEntry`).

Since the code calling for such signal/hook procs is compiled away if they're not present, there would be no impact on performance by adding such a feature.

I can look into a norm-implementation of such a thing and submit a PR as I did for #127 , but I thought it might make sense to first even ask if such a feature is desired or if you see problems with it before I waste time on something unwanted.