Added new config option with readme documentation

[MattMakes]

Adding this to allow us to configure an http/https agent for keep alive configurations. We've seen a large number of sockets being opened up by this library when used by our projects and believe it's due to not having configs available to update the request node module (which is now deprecated, btw, but that'll have to be another PR..).

The example provides an usage example - but the simplicity of it would allow for using something more complex like this too: https://github.com/node-modules/agentkeepalive

Please let me know if you have questions or concerns.

[bparmar-splunk]

HI @MattMakes,
This PR contains code conflicts with current branch codebase.
Could you please resolve the conflicts and push it again?

Also, it would add more clarity to this change, if you add test case supporting this use-case.

Thank you.