title: Equivalent of publish subject in RxJs
body: <p>I notice that publish subject is not implemented in RxJS. Is there a way to simulate something like it ?
  I want to receive only the data that is sent after I subscribe, without the last value. Exactly the behavior of an event emitter in angular 2.
  All the other subject types seem to store the last value and send it.</p>
  
  <p>With regards</p>