Changed markdown, rtf, and HTML-based templates accordingly.
This allows you to set `toc: true` in the metadata; this
previously produced strange results in some output formats.
Closes#2872.
For backwards compatibility, `toc` is still set to the
toc contents. But it is recommended that you update templates
to use `table-of-contents` for the toc contents and `toc`
for a boolean flag.
