Make superagent replacable with other libs like isomorphic-fetch

[thomasfl]

Making the XmlHttpRequest library replacable could potentially make this beautiful little library into bloatware. You would need a set of drivers for the most common XmlHttpRequest clients like superagent and fetch.

[mallison]

Hi!

I was thinking to make it pluggable so the user can pass it whatever ajax library they like -- as long as it conforms to a minimal API (which may mean they need to wrap their favourite library). So the onus is on the user to make their agent of choice fit, rather than redux-rest having to support the most common libraries. I don't think this would add bloat.