Add ToC generation to gfm output

[ExplodingLemur]

When converting from MS Word (docx) to Markdown using gfm as the output format, the --toc flag has no effect. When using markdown_github however it generates a table of contents from the section headings in the document.

Feature request: Add table-of-contents generation to the gfm output format.

Example of commands used:
pandoc Blah.docx -f docx -t gfm -s --toc --extract-media=Blah -o Blah.md No ToC generated
pandoc Blah.docx -f docx -t markdown_github -s --toc --extract-media=Blah -o Blah.md ToC is generated

[mb21]

Indeed, the template is correct, but the commonmark writer doesn't populate the $toc$ and $table-of-contents$ variables...