Process page breaks

[erlendp]

Seems like a useful feature from NicoJuicy that was closed on the original OfficeDev repo when MS stopped maintaining the project.

[djbios]

When this feature will be merged? It can save me :)

[dpomBF]

@NicoJuicy, wouldn't line 416 be
return validElements.Any(dl => dl != null && dl.Attribute(W.type).Value != null && dl.Attribute(W.type).Value == "page");
instead of
return validElements.Descendants().Any(dl => dl != null && dl.Attribute(W.type).Value != null && dl.Attribute(W.type).Value == "page");?