For https://github.com/rstudio/sass/pull/30

sass-printing.Rmd

---
title: "Untitled"
output: 
  html_document:
    highlight: tango
---

```{r}
library(sass)
bg_black <- list(
  list("body-bg" = "black"),
  "body{background-color: $body-bg;}"
)
color_white <- list(
  list("color" = "white"),
  "body{color: $color;}"
)
```

Printing by default embeds the CSS as HTML.

```{r}
sass(bg_black)
```

Printing with `class.output='css'` and `comment=''` allows for syntax highlighting.

```{r, class.output='css', comment=''}
sass(bg_black)
```

Also works with `sass` objects:

```{r}
as_sass(color_white)
```

```{r, class.output='css', comment=''}
as_sass(color_white)
```