Introduce data-element

This release introduces a helper attribute, data-element, which can be used to remove unbound elements from the document. Documentation here: https://github.com/PhpGt/DomTemplate/wiki/automatically-remove-unbound-elements

What's Changed

• data-element by @g105b in #439
• Remove data-element attribute by @g105b in #440

Full Changelog: v3.2.0...v3.2.1