Adds support for default tags at tracer level

There are many use cases where we want to pass tags to all our spans based on enviroment parameters (e.g. instance id). They are static values and can perfectly fit in the tracer at bootstrap time.

Zipkin-go does include this already: https://github.com/openzipkin/zipkin-go/blob/master/tracer.go#L17

cc @adriancole 