Add remark-flexible-toc to list of plugins

Closes GH-1339.

Reviewed-by: Remco Haszing <remcohaszing@gmail.com>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>

doc/plugins.md

@@ -181,6 +181,9 @@ The list of plugins:
 * 🟢 [`remark-flexible-paragraphs`
  ](https://github.com/ipikuka/remark-flexible-paragraphs)
  — add custom/flexible paragraphs with customizable properties
+* 🟢 [`remark-flexible-toc`
+ ](https://github.com/ipikuka/remark-flexible-toc)
+ — expose the table of contents (toc) via Vfile.data or an option reference
 * 🟢 [`remark-frontmatter`
  ](https://github.com/remarkjs/remark-frontmatter)
  – support frontmatter (yaml, toml, and more)