Customise output of R code using a knitr output hook

[cderv]

About output hook, some example could be part of the cookbook

• Customise error output to change color of error message in html (SO QA)

knitr::knit_hooks$set(error = function(x, options) {
  paste0("<pre style=\"color: red;\"><code>", x, "</code></pre>")
})

[cderv]

Introduction added in c7c2e23

[cderv]

About the example above, this is now way easier with the new class.* options

```{css, echo = FALSE}
.rerror {
  color: red;
}
```


```{r error=TRUE, class.error="rerror"}
stop("test")
```

[cderv]

We can close and prefer #38