new function all_labels()
roxygenize
this does not make much sense; why not simply call the argument 'options' instead?
wrap.list() and parse_chunk() return x if its length is 0
is_blank() returns a scalar logical value now; if x is empty, it returns TRUE
no need to use all() on is_blank() now
no need to str_c() here, because it will be done later
length(output) will never be 0, because str_c() will at least return ''
tidy code only if it is not empty
do not return '' early if the chunk is empty; proceed so that potential chunk hooks will be run
use library(rgl) instead of require('rgl') because the former will throw an error if rgl is not installed
the demo was broken due to the commit rstudio/shiny@3f59a7d, use plain reactive() instead
render_html() should set the out.format to be html; fixes #447
render_markdown() does not have to set highlight to be FALSE; setting out.format=markdown is enough to avoid syntax highlighting in R
must load the parser package explicitly for highlight::highlight() to work, otherwise we get this error:

Error in match(data$token, grammar_symbols$token) :
  object 'grammar_symbols' not found
use the chunk label as the id of the div
news about #447
factor out a few elements in inline_exec() so that it will be reusable for #397
use the inline value only if the inline code returns a visible value
da63b0d0e for #447 is not a complete fix: prompt=TRUE should apply to Rmd too