[ENH] add commonmark-extras extension to the Markdown plugin

Description

I've added commonmark-extras extension which "is a collection of useful GFM extensions and utilities for the league/commonmark project" to the Markdown plugin.

From the README.md file:

Adding this extension to your project will automatically register these sub-extensions:

Extension	Purpose
league/commonmark-ext-autolink	Automatically creating links to URLs and email address (without needing the <...> syntax)
league/commonmark-ext-smartpunct	Intelligently converts ASCII quotes, dashes, and ellipses to their Unicode equivalents
league/commonmark-ext-strikethrough	Adds support for ~~strikethrough~~ syntax
league/commonmark-ext-table	GFM-style tables
league/commonmark-ext-task-list	GFM-style task lists - [x] Like this

Sincerely,

Gour