Add markdown

[statnmap]

• add gm_markdown_body() for a body in markdown
• Markdown text is transformed into HTML using {markdown}
• I updated doc and DESCRIPTION using attachment::att_amend_desc() as some deps in
  DESCRIPTION were not necessary anymore apparently

attachment::att_amend_desc(extra.suggests = "covr", pkg_ignore = "base") could be used after each dev modification.

[jimhester]

Thanks for working on the PR. I think if we were going to add markdown support we should use the rmarkdown package rather than the markdown package.

Please remove the unrelated DESCRIPTION dependency changes.

[statnmap]

I do not see in {rmarkdown} which function I can use to render a text fragment, other than creating a temporary external file to render with html_fragment() .
Did I miss something in the package?