title: Programmatically (un)register to event with Angular 2
body: <p>I have a directive that listens to some events using the <code>host</code> property of the <code>@directive</code> decorator. This works great, but I don't need all events at all times.</p>
  
  <p>For example, I'm only interested in <code>(document: mouseup)</code> at certain times (namely after a mousedown on my element). </p>
  
  <p>What is the angular way to register and unregister to events dynamically?</p>