my RStudio project options
center the body; base font-size 13px; more line height
max-width for <pre> is 100% to make sure there is no ugly right margin

only <code> with a class has a gray background; this means ```foo also has the background color

closes #45, although it is not perfect yet
use the Github highlighting theme (#45)
now we no longer need the hack vignette.css in knitr
tweak the mathjax_embed feature
\VignetteEngine{knitr::knitr}
add another vignette to show the HTML output in the web browser