xaringan 0.21

• When an image is empty (e.g., ![]()), the output format option self_contained: true of xaringan::moon_reader fails to encode the image (thanks, @Mosk915, #302).

• css argument in moon_reader() now correctly support .scss and .sass files following their support in base format rmarkdown::html_document() in rmarkdown 2.8 (thanks, @abichat, #310).