Add remark-extended-table to list of plugins

Closes GH-886.

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

doc/plugins.md

@@ -97,6 +97,8 @@ The list of plugins:
  — embed local images as base64-encoded data URIs
 * 🟢 [`remark-emoji`](https://github.com/rhysd/remark-emoji)
  — transform Gemoji short-codes to emoji
+* 🟢 [`remark-extended-table`](https://github.com/wataru-chocola/remark-extended-table)
+ — extended table syntax allowing colspan / rowspan 
 * 🟢 [`remark-extract-frontmatter`](https://github.com/mrzmmr/remark-extract-frontmatter)
  — store front matter in vfiles
 * 🟢 [`remark-first-heading`](https://github.com/laat/remark-first-heading)