Allow custom Http client options

[rosswintle]

Thanks for the awesome package!

I was using this locally and was getting curl errors:

cURL error 60: SSL certificate problem: unable to get local issuer certificate

After realising that I couldn't modify the verify option with a globalRequestMiddleware I decided to add a withOptions method to enable it.

I've updated the README accordingly.

Testing

I spent several hours (I know!) trying to find a way to test this, but with how I'm setting the options there does not seem to be a way that I could find. I source-dived, and xDebugged, but the option seems to be set on the curl instance and with the Fake I don't think we even get to that depth in the test.

I tried to create a mock, but this felt like we were just testing that the new method does what the new method does.

Happy to take your advice here.

Thanks.