Allow extensions to be added to the Client like headers / auth

[KalleDK]

Like you can add headers / auth and a base_url, I would love the ability to add extensions to the Client, and have them either merge (like headers) or overwrite (like auth).

This arrises from using base_url with ip and sni_hostname for https.

Right now my workaround is a wrapper that adds the sni_hostname to all requests, but it feels silly to do as the rest (base_url, auth and headers) is already preapplied via the Client