Update ExternalTemplateSource.js

Added the option to pass a writeable observable to the template binding, in order to get notified when the template was loaded.
example:

<script> 
myWriteableObservable = ko.observable( false ); 
</script>


<div data-bind="template: {name: 'mytemplate', loaded: myWriteableObservable } "></div>
<div data-bind="visible: myWriteableObservable">Template was loaded</div>