title: KnockoutJS subscribe to multiple observables with same callback action
body: <p>I've got a model class in KnockoutJS which has multiple values that I'd like to subscribe to.  Each subscription will perform the same task, like so:</p>
  
  <pre><code>function CaseAssignmentZipCode(zipCode, userId, isNew) {
    var self = this;
    self.zipCode = ko.observable(zipCode);
    self.userId = ko.observable(userId);
    self.isNew = isNew;
    self.isUpdated = false;
  
    self.zipCode.subscribe(function () { self.isUpdated = true; });
    self.userId.subscribe(function () { self.isUpdated = true; });
  }
  </code></pre>
  
  <p>Is there a way to combine these two calls to subscribe, so that I can use one subscription to 'watch' both values?</p>