Functional Options support

[italolelis]

Hello,

Thank you a lot for this amazing library, it's really useful and well thought.
How would you feel about supporting function options? Something like this:

hystrixClient := heimdall.NewHystrixHTTPClient(
	timeout,
	heimdall.WithCommandName("MyCommand"),
	heimdall.WithHystrixTimeout(1100),
	heimdall.WithMaxConcurrentRequests(100),
	heimdall.WithErrorPercentThreshold(25),
	heimdall.WithSleepWindow(10),
	heimdall.WithRequestVolumeThreshold(10),
    heimdall.WithHystrixRetryCount(2),
	heimdall.WithHystrixRetrier(heimdall.NewRetrier(heimdall.NewConstantBackoff(10, 5))),
)

Is this something you'd like to see in the lib? If yes I can send a PR with it.

Thanks again!

[rShetty]

Hey @italolelis I think it makes sense. Can you go ahead with the PR

[italolelis]

Closing this issue as #33 is already merged 🎉