respect options(warn=-1); fixes yihui/knitr#610
closes #40: the required change could be really simple
cosmetic
for a large number of expressions, evaluating new_output_handler() again and again can take some time and also unnecessary, so just create a default copy beforehand and use it
closes #41: when the value handler has more than one argument, the second argument is 'visible'
test case for #40
test case for #41
news for #40
news for #41
roxygenize
bump version