Log activity for subjects that aren't Eloquent models

[jacksleight]

I'd love to be able to use this to log activity for other types of object, not just Eloquent models.

My current use case would be logging activity against Statamic Entry objects (Statamic does have an Eloquent driver but that's optional, I'd like it to work with the file based setup as well).

Another use case could be logging activity against local files, like downloads or something.