Better Markdown Integration

[brodieG]

As discussed in this thread, markdown integration is possible but a bit hacky. See also the vignette. Ideally there would be either a format or pager or something that does the direct output in the correct format. Remains the issue of how to specify the "asis" knitr chunk option.

More broadly, once the correct output format is setup (maybe even self detecting that running under markdown), is it possible to intercept viewer calls in some way so that the html output shows inline as plots do without any additional work required. This would likely fall onto whatever is generating and running the markdown document (e.g. reprex), though not sure about this as I haven't dug into it at all.