capture.output() has already considered withVisible(), and we do not need to redo it
add a 'width' argument so we can control the width of the text output
a news item for the 'width' argument of renderPrint()
fixes #429, which is yet yet another WAT of RJSONIO

perhaps we really should consider switching to jsonlite...
\emph cannot be used in \code{}: only \var and \link are allowed
Merge remote-tracking branch 'origin/pr/485'

Conflicts:
	NEWS
add a 'width' option to selectizeInput() and sliderInput() to specify the width of these widgets
roxygenize
validateCssUnit(width) for selectize and slider