Docx reader: Single-item headers in ordered lists are headers.

When users number their headers, Word understands that as a single item
enumerated list. We make the assumption that such a list is, in fact, a header.
Docx reader: Rewrite rewriteLink to work with new headers.

There could be new top-level headers after making lists, so we have to
rewrite links after that.
Docx Reader: Add tests for numbered headers.