[feature request] add server configure

[unship]

run bazel run //src/main/java/braintree:bazeldeps myserver junit:junit:4.12

get

maven_jar(
    name = "junit_junit_4_12", 
   artifact = "junit:junit:jar:4.12"
   server = "myserver",
)

[pgr0ss]

I'm not really maintaining this project anymore. That said, if you want to try to implement this, I can probably merge it in.