Support MarkDown in the various argument and tool usage annotations

[nh13]

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Could use: https://github.com/planet42/Laika

Uses are to produce html, pdf, and text documentation for a given command line tool or Dagr.

[nh13]

The current thinking is as follows.

Re-implement the HTML renderer found here: https://github.com/planet42/Laika/blob/master/core/src/main/scala/laika/render/HTML.scala

Adapt the HTML test class here: https://github.com/planet42/Laika/blob/master/core/src/test/scala/laika/render/HTMLRendererSpec.scala

The idea would be to produce a plain-text renderer, suitable for a terminal output. Terminal codes (for coloring, or emphasis, or the like) could be used, either specified in the input markup or based on convention (like we do automatically now).