R HTML uses more info in opts_knit$get('header'), per request of Erich Neuwirth
a cosmetic change and roxygenize
Revert "closes #601: results='asis' implies comment=NA"

This reverts commit 7a89374219ff5fd4823a9d2ce951fac21ddc6018.

results='asis' will affect warnings/messages output as well
bump version