Set up MarkdownCodeBlockFormatter

[angelikatyborska]

I wrote an Elixir formatter plugin that is able to format Elixir code in Markdown files. @jiegillet @neenjaw what do you think?

(I'll publish it to hex before merging)

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Automated comment created by PR Commenter 🤖.

[angelikatyborska]

Part of the problem is that there is no one Markdown standard 😞 This syntax:

[]: # (elixir-formatter-disable-next-block)

It's an abused reference-style link. More generically, it's:

[reference-name]: https://example.com (Some link title)

Now, I don't know if all Markdown implementations support reference-style links, but Exercism definitely depends on them. I'm pretty sure that # must be a valid link href, and elixir-formatter-disable-next-block is a valid link title. The only thing I'm unsure about is an empty reference name. I didn't see any problems so far. It could also be non-empty (could be anything), but configlet generate moves all non-empty link references to the bottom of the markdown file which messes up with my intended use 😁