feature: allow binding directly to data-element

[g105b]

Previously, the data-element attribute could be added to any element, and that element would be removed if it did not have any other bind operation performed on it.

For example, on <p data-element data-bind:text="message"></p>, if there was no bind to the message key, such as bindKeyValue("message", "Hello, World!");, the element was removed.

This new functionality means that you can add a value to the data-element attribute, and it can now be bound like any data-bind attribute. For example:

<p data-element="error">There was an error</p>

function go(Binder $binder):void {
  $binder->bindKeyValue("errorMessage", true);
}

[g105b]

Updated the docs at https://github.com/PhpGt/DomTemplate/wiki/Automatically-remove-unbound-elements-with-%60data%E2%80%90element%60