Ability to apply to model with existing slug property

[NightJar]

In the case that a model has already been defined with a slug property, the addition of URLSlug is unnecessary.
Would it be possible to apply the Slug extension to such a model to utilise its generation, synchronisation, and 'state' functionality, without it also necessitating the existence of URLSlug, instead using a property of arbitrary name to store and check the generated value?

The outcome of this issue may in fact be only to update the documentation to make clear that this module does not support such a use case. However I think something could probably be achieved - but it should be balanced between code complexity and maintainability (e.g. would such a feature be rendered un-upgradable to SS5?)

Thanks to @ramono for bringing this up through a slack thread :)