Support of OpenXml 2.12.0+
This release bring back support to NET 4.6+, NET Standard 1.3 and NET Standard 2.0.
Use new API OpenXmlCompositeElement.AddChild(OpenXmlElement)
to add children in the correct order per schema, introduces in OpenXML 2.12.0. Previous version of Html2OpenXml was using internal code via Reflection.