check if a source document is using the Sweave noweb syntax
if an Rnw document is using Sweave Noweb syntax, pop up a message box as reminder
this trick of error hook is more of a hack; use stop_on_error instead
an example of converting the official Sweave example
remove extra lines of @ in Sweave documents
roxygenize
mention the message box to remind Sweave2knitr() in the news
reword message
escape_latex() can also preserve spaces now and messages in LaTeX will keep spaces
if chunk option include=FALSE, use stop_on_error=2L; fixes #453
bump version