Zipkin updates + Axios

* Zipkin instrumentation now has a separate axios package: https://github.com/openzipkin/zipkin-js/tree/master/packages/zipkin-instrumentation-axiosjs
* Zipkin has a Koa implementation: https://github.com/uswitch/zipkin-instrumentation-koa
* Zipkin also now has different options as of 0.19.0