Support a configuration file for defaults

[yschimke]

$ cat ~/.zipkinrc
ZIPKIN_SINK=scribe://localhost:1463
ZIPKIN_UI=https://zipkin.company.com/zipkin/traces/{traceid}

URL in same format as #52

Would be nice if client tools could support zipkin without asking for complicated URLs to be entered.

[yschimke]

I might implement these myself in another project, if I do I'll link here if you want to reuse the code.

[yschimke]

Based on these a Default zipkin reporter factory could provide the reporter and a Function<Span, URI> for clients to use without additional configuration