Add a DocumentReader implementation for GH Markdown

[markpollack]

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Github Markdown is a popular format that provides ready structural hints as to document content, e.g paragraph, table, etc and many document formats can be converted to markdown, for example pandoc can convert asciidoc to markdown which makes it easier to work with.

I've used https://github.com/commonmark/commonmark-java and the GH extension with success in spring-cli.