Add an `init` hook

[purepear]

Having a way to alter the options before initialising the request would be helpful. Currently there's no easy way to alter the searchParams or urlPrefix in a hook. For example, sanitisation/modification of searchParams is only possible through the beforeRequest hook by "deconstructing" and then constructing and returning a new request AFAIK. I think sth like the init hook of got will be useful in these(and other) situations.