Use a standards-compliant Markdown implementation

This project implements the CommonMark standard in php: https://github.com/colinodell/commonmark-php

This is an initial attempt at using a the CommonMark standard by leveraging the commonmark-php library.

Commonmark is just yet another variant of Markdown. My goal for the D8 release is to make it easy to use more or less any Markdown PHP variant.

We needed this to be able to support responsive images while using markdown, in particular we needed this to be able to output a data attribute on an image tag, I wrote a short blog post describing how we did it, https://attiks.com/Zkt

Regarding the "standard" markdown, there is no such a thing, I admit it is a great initiative, but for the moment it is just another variant, most implementations have nice-to-have extras and I wonder if the "standard: one will add them all.

http://talk.commonmark.org/t/are-there-any-standards-compliant-php-imple...

Per #2191525: [PP-1][policy, no patch] Extend Markdown coding standards to support API module (DOXYGEN) Drupal's coding standards are likely going to embrace http://commonmark.org/ so this would make sense to go with the flow and have a common standard.

A better way around this would be to support both CommonMark and Markdown-php, but have a configuration option in the filter settings to select which processor to use.

I have now added an setting to Markdown filter to select between PHP Markdown and CommonMark to 8.x-1.x. Please install and test.

PHP Markdown can be installed either the old way in the libraries folder, you then need the libraries module for it to be loaded.

PHP Markdown and CommonMark can both be installed with composer and will then be autoloaded. (Composer is handy for developers but I don't really understand how others are supposed to do this. This seems to be the recommended way in Drupal 8 however.)

Any other markdown libraries we should add as options?

Happy to here suggestion for a better way to implement this.

GitHub has switched over to GFM with ComonMark.

Can we get this backported for Drupal 7?

Project page could use a description update as well.