Support link to custom model

[weber-s]

Hello there and thank you very much for this plugin!

We have a similar implementation that support also link to custom model, that are not directly tied to a page but have method like get_absolute_url.

Would you be interested in a similar feature? We can do a PR if wanted :)

Technical note:
In our implementation, the block can add custom field like

            linked_myapp_news = SnippetChooserBlock(
                target_model=News,
                required=False,
                label="Select a news",
            )

and as many as wanted, and the similar toggle for the dropdown chooser apply to hide/show the snippetchooser.