allow passing request wrapper option

[jstewmon]

request has a robust set of options that are currently inaccessible. This PR allows FB users to supply their own request wrapper, so that all request options are configurable.

allows #100 to be addressed by library users by supplying their own agent

[dantman]

What request options other than those in #100 are useful?

Request refuses to play nice with libraries following semver, it's bloated making our library+deps larger than it needs to be, and the bloat is also a potential source of future security issues caused by things we don't need. Rather than exposing request options the current goal is to replace request entirely with a much more lightweight library.

[jstewmon]

I'm only concerned with being able to pass an agent and set the time option, so that I can instrument and optimize connection handling. Getting the timing info will require another change to expose the response object somewhere.

I suppose you're reluctant to make an option for request b/c swapping the client library would become a major semver change?

Do you have an alternate library in mind? I can make another PR that moves to the library you want and make it configurable.

[dantman]

I'm reluctant because it would make some usage of the library request-specific and would make upgrading difficult when we do switch libraries.

I haven't done a complete comparison yet, but when I did a quick look over node.js http libraries, needle seemed like it might be a good choice. Few deps, promises, recent commits, and no fancy version restrictions.

[jstewmon]

I think needle is a fine choice. I implemented it in #125