Use markdown in roxygen comments

[spencerpease]

Description

Starting from version 6.0.0, roxygen supports markdown markup within most roxygen tags. Roxygen uses the commonmark package, which is based on the CommonMark Reference Implementation to parse these tags.

This would help make comments more concise and readable, since \emph{foo} could be replaced by **foo**, \code{bar} would become bar (just in backticks), and \code{\link[=func]{func()} becomes [func()].

The roxygen tags @md and @noMd to parse roxygen chunks with/without markdown for a single chunk.

References

https://cran.r-project.org/web/packages/roxygen2/vignettes/markdown.html