Support Word autonumbering feature

[tarleb]

Word has different numbering styles that can be used to number sections. It would be great if the docx reader could detect this and add the relevant information, e.g. the unnumbered class to headings.

We may also want to use that feature in the writer.

Cc: @frederik

[tarleb]

Example: article-numbered.docx

Same example but unnumbered for comparison: article-unnumbered.docx