Add option to output Critic Markup

[alerque]

In the current iteration Critic Markup is treated as a one way street — touch it and it's gone. You can output to HTML or LaTeX. Outputting to Markdown only adds inline HTML tags to the Markdown option.

There should be an output option for outputting Markdown with Critic Markup syntax back in place. In other words it should be possible to round-trip between formats.

See also jgm/pandoc#5430.

[ickc]

Are you only talking about round-tripping markdown, or the input can be of any format (like #4 is trying to do) which has plain text CriticMarkup syntax?

[alerque]

For me, primarily round-tripping Markdown. I could see the utility for the latter, but the former is my current use case.