Global proxy setting?

Hi! Love your library, using it all over the place. I had a quick Q though:

I use [Charles](http://www.charlesproxy.com/) as a local request inspector, which I end up routing in your library using `.proxy('http://localhost:8888').strictSSL(false)` , but I was wondering if there is a way to set this globally, and perhaps conditionally based off the environment variable? I'm finding I'm using it on a lot of my requests, and am needing to go and clean them all out when I push my code back up to production.

Lemme know :) Thank again for such a great library!

Start by tracing the existing `.proxy('http://localhost:8888').strictSSL(false)` request path and checking how this Node.js library accepts configuration. Define the desired global and environment-variable behavior, including how production avoids the local proxy, then verify requests use it without per-request proxy calls.

Written by the indexing model from the issue text.