Add support for horizontal lines in documents.
In DOCX, such lines are defined using the legacy VML definitions:
<w:pict w14:anchorId="324D5836">
<v:rect id="_x0000_i1025" style="width:0;height:1.5pt" o:hralign="center" o:hrstd="t" o:hr="t" fillcolor="#a0a0a0" stroked="f"/>
</w:pict>
The definitions above and the <hr/> HTML tag are currently not supported and skipped on import, leading to missing horizontal lines in the document.