Hostname is certainly relevant. We have two options for the `purge` method's interface:
1. `purge($path, $hostname = null)`
2. `purge($url)`, where `$url` can be either a path or an absolute URL. If a path is given, the default hostname is used. If an absolute URL is given, that URL's hostname is used.

I chose the second option, because I think it's neater. Do you agree? If so, I'll add some explanation to the method docs.