Allow for a custom request object to be passed to express-request-proxy

[talawahtech]

It would be great if we could pass in a custom request object for the proxy to use. This would allow us to configure advanced request options or use other libraries that wrap request to provide additional functionality e.g. https://github.com/FGRibreau/node-request-retry

In my case I am proxying and caching a somewhat unreliable API so I would like to be able to retry it a couple of times with an exponential backoff strategy if I get a 5XX response.

[thinkkevin]

I would second that.