[FR] allow specification of `TOC` and related options

[vjd]

I understand that we can use pandoc_options in weave() to request a table of contents and other options such as depth. It would be useful to have support for these additional features. An example specification from rmarkdown is here.

---
title: "Habits"
output:
  html_document:
    toc: true
    number_sections: true
    toc_float:
      collapsed: false
      smooth_scroll: false
---