`url` should be a peer dependency

[samhh]

Or we can stop using it as per #15.

[OliverJAsh]

Even for Node? For Node it's built-in.

[OliverJAsh]

To answer my own question: it's needed when someone tries to use this library outside of Node i.e. in the browser.

In that case, I'm not sure whether we should list url as a dependency or expect consumers to provide it themselves.