Add support for passing an ng-template reference in the [template]

[ozkoidi]

It would be useful to be able to pass the reference of an ng-template to the [template] property. This would allow us to have something like this

<ngx-spinner [template]="loadingTemplate"></ngx-spinner>

<ng-template #loadingTemplate>
  <div>
    <span>Uploading...</span>
    <progress [value]="uploadProgress"> {{ uploadProgress }}% </progress>
  </div>
</ng-template>

I think this feature was already proposed on some issues and other Angular libraries such as ngx-loading already support it.