New module for CommonMark documents

[gemmaro]

Hello,

This adds a new module for the CommonMark and closes #639. This is also a continuation from #642 to fix a minor point.
Note that this is not intended to replace the Text module with the markdown option since the CommonMark doen't have additional features such as footnotes or tables.

Thank you,

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TODOs before mark as ready:

• Add tests
• Add mentions to this module in the Chooser module and others (such as readme).
• Remove unrelated changes (?)
• Update issue number in the news
• Fix Docker image prerequisites