Use markdown in roxygen doc

[jgabry]

I converted all of the doc for the loo package recently to use markdown syntax in the roxygen doc (stan-dev/loo#108) and I think it's worth doing for bayesplot. The roxygen2md helps but then there's a bunch of manual editing required afterwards. I'll do it when I have time, but in the meantime we can also start the process by changing individual files that we're editing for other reasons (e.g., #190 does this for the bayesplot-colors.R file).

[jgabry]

Actually once I started it wasn't too bad so I kept going and and didn't take as nearly long as I thought. Almost finished over on the markdown-in-roxygen branch.