Provide convenient access to the default client config

[mliarakos]

Due to akka-js/shocon#55 adding client settings to the lagom.js client config is difficult. Lagom.js should provide access to the default client config so it can be used by user, similar to Akka.js:

lazy val conf =
  ConfigFactory
    .parseString("""
    akka {
      loglevel = "DEBUG"
      stdout-loglevel = "DEBUG"
    }""")
    .withFallback(akkajs.Config.default)

lazy val system = ActorSystem("yourname", conf)

[mliarakos]

Fixed by #22.