Add beforeLoad callback

[nezuo]

It should be possible to modify the data of a document before it loads. For example, you could implement reconciliation (think ProfileService:Reconcile) or decompression of the document.

Some open questions:

• Should this callback run before or after migration? Should there be a callback for both?
• Should this callback be global or per collection?

I'd also like to add a reconciliation example to the docs after this feature is added.