Apply custom-attributes from the view-model

I couldn't find any info on this topic in the docs and i'm not sure if the template engine supports this use-case today but:

I'm trying to init and apply a custom attribute from the view-model onto a referenced HTMLElement.
Does the templateEngine support this? Else I'd like this to be a feature request.

This is what I have tried so far:

``` Javascript

// create a new instance of our popover custom-attribute.
// target is our referenced HTMLElement
let validationPopover = new Popover(target);
this.templateEngine.enhance({
element: target,
bindingContext: validationPopover
});
```

Thanks
-- Mike

Start at the templateEngine.enhance({ element: target, bindingContext: validationPopover }) example and inspect how referenced HTMLElements and custom attributes are currently handled. Determine whether the existing enhancement entry point supports this view-model flow; if not, define the feature behavior and add coverage showing that the custom attribute is applied to the target element.

Written by the indexing model from the issue text.