Investigate supporting roxygen2 documentation via markdown

[gfinak]

Need to see what's involved in doing this and how this could be incorporated first.

[gfinak]

It seems the maxygen package can do this https://github.com/gaborcsardi/maxygen
but it registers callbacks for roxygen that cannot be unregistered without a restart of the R session, so
mixing regular R-based roxygen and maxygen may not be possible at this time.
ropensci/software-review#230

[gfinak]

As noted by @wlandau, adding Roxygen: list(markdown = TRUE) to the DESCRIPTION will enable support for markdown based roxygen.