Initial SourceWithContext, FlowWithContext, FlowWithContextOps.

(based on https://github.com/akka/akka/compare/master...jrudolph:jr/withContextOps)

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -7,7 +7,7 @@ package akka.stream.scaladsl
 import java.util.concurrent.CompletionStage
 
 import akka.actor.{ ActorRef, Cancellable, Props }
-import akka.annotation.InternalApi
+import akka.annotation.{ ApiMayChange, InternalApi }
 import akka.stream.actor.ActorPublisher
 import akka.stream.impl.Stages.DefaultAttributes
 import akka.stream.impl.fusing.GraphStages
@@ -215,6 +215,11 @@ final class Source[+Out, +Mat](
 
       combineRest(2, rest.iterator)
     })
+  /**
+   * API MAY CHANGE
+   */
+  @ApiMayChange
+  def startContextPropagation[Ctx](f: Out ⇒ Ctx): SourceWithContext[Ctx, Out, Mat] = SourceWithContext(this).mapContext(f)
 }
 
 object Source {