Extract converters into plugins

Created merge request

I think this is a brilliant idea.

However there are a number of code quality issues being reported by PHPCS and PHPStan, so those would need to be fixed.

Unfortunately the bigger issue is that I don't think new development is happening on the 1.x branch, and a number of new features have been implemented on the 2.x branch.

I would recommend:

• Rebase against 2.x, and fix any conflicts.
• Add a plugin for the "Markdown Smörgåsbord" flavor. Could this be extended from the GitHub flavor, since its a superset of its features?
• Consider additional cases to explicitly test the plugin system.

Setting to "needs work".

I have only addressed the code quality issues for now because there isn't even an alpha release for 2.x. I'll be happy to reroll once 2.x is released.