Support replyWhileStopped in BackoffSupervisor

[ianclegg]

Akka for Scala introduced a replyWhileStopped feature to the BackoffSupervisor last May. One case where its useful is when using the Ask Pattern. The feature means the Ask gets an immediate response when the child is stopped as part of a backoff, this is often preferential to having the Ask forwarded to DeadLetters by the BackoffSupervisor, thus relying on an Ask timeout. See akka/akka#21213

  Backoff.onFailure(childProps, childName, minBackoff, maxBackoff, randomFactor)
    .withManualReset
    .withSupervisorStrategy(
        OneforOneStrategy(){
        case e: GivingUpException => Stop
        case e: RetryableException => Restart
    })
    .withReplyWhileStopped(TheSystemIsDown)

Obviously adding the feature will increase the API surface slightly, and may involve the addition of a parameter to the BackoffSupervisor constructor (which is public) - though most people will not be using the constructor directly because the factory methods are the general way to go. I'm happy to string together some commits, tests and doc update to get a PR moving, what do you think?