Add codec based on env var JAEGER_PROPAGATION

[jpkrohling]

The Java client has a feature where codecs can be specified via the env var JAEGER_PROPAGATION. Here's its description:

Comma separated list of formats to use for propagating the trace context. Defaults to the standard Jaeger format. Valid values are jaeger and b3

This issue is to bring the feature to the Go client. Currently, only jaeger is automatically set. The following code can be used to add the Zipkin to the tracer options:

	zipkinPropagator := zipkin.NewZipkinB3HTTPHeaderPropagator()
	injector := jaeger.TracerOptions.Injector(opentracing.HTTPHeaders, zipkinPropagator)
	extractor := jaeger.TracerOptions.Extractor(opentracing.HTTPHeaders, zipkinPropagator)

And here's the relevant code in the Java client:

https://github.com/jaegertracing/jaeger-client-java/blob/master/jaeger-core/src/main/java/com/uber/jaeger/Configuration.java#L437-L462

[yurishkuro]

the big downside in Go if we support this feature is that the config module starts to depend on all other modules, whether the end user needs them or not. I think it is an anti-pattern in Go.