Expand interface to support url/protocol specific results.

[tracker1]

I was just checking before I create a new npm module, I wanted to do something similar, but have a URL passed in, then check against the NO_PROXY list, and from there check for a protocol specific proxy, and fallback to ALL_PROXY and from there returning null if no match.

I already created the no-proxy module to check the first part, and was about to create a new module for the get-proxy. Would you be open to a PR?