feat(blocks): callback

[Mte90]

Ref: #51

So in my investigation I discovered that the issue is because Unfold register a custom node, CaptureNode so it isn't possible to detect what is the real block as it isn't a Django one.

With this code we are adding a new callback so the dev can implement a way to detect those blocks.
The readme include the code for Unfold theme as example.

[Mte90]

I need also to add:

        result.add('breadcrumbs')
        result.add('nav-sidebar')
        result.add('usertools')

In this callback to fix the warnings, as I can see in this case they are just blocks https://github.com/search?q=repo%3Aunfoldadmin%2Fdjango-unfold%20usertools&type=code but I think that they are not loaded always just present if the template it will just calls them.