Make defaults configurable

[jcchavezs]

There are cases where you want to plug this library as a replacement for fetch but transparently. In such cases one can't take advantages of the configurable features like retries, retryDelay or retryOn because original fetch signature does not accept such parameters. Having the possibility to pass those values on require will allow users to plug this fetch like any other fetch.

Related to openzipkin/zipkin-js#485

Ping @jonbern @petermetz