Allow to configure Guzzle options / use a proxy to ensure static IP address

[holtkamp]

Currently the Bunq API has a "fixed IP" requirement. When running an application from a "cloud" environment, the IP typically changes. By configuring Guzzle to use a proxy service that enables a static IP, this can be circumvented.

For example:

• Fixie
• Proximo

So it would be nice to have some control over the Guzzle configuration, or at least be able to indicate a proxy as described here 😄