Add ability to set configuration all at once

[davidhemphill]

Sometimes you just want to configure Zttp all at once or you need to set an option on the underlying Guzzle client that Zttp doesn't expose:

Zttp::configure([
    'headers' => [
        'Accept' => ['text/xml'],
    ],
]);

[adamwathan]

Feel like renaming this to withOptions to match the naming style of the other methods?

[davidhemphill]

Done! This actually came to mind randomly yesterday too.

[adamwathan]