Table of contents (only for WordDocumentBuilder at this time)

[goeh]

This pull request adds support generating Table of Contents with the toc() tag.

paragraph(font: [size: 20.pt, bold: true]) {
    text "Table of Contents"
}

toc()

pageBreak()

It's not 100% complete because (as usual) it's much harder to do this in PDF. I'm not sure I could do it. If someone with more PDF/PDFBox experience can step in I would be very thankful.