Display engine images in the exercise caption

[schloerke]

PR task list:

• Update NEWS
• [NA] Add tests (if possible)
• Update documentation with devtools::document()

remotes::install_github("rstudio/learnr#429")
rmarkdown::run("learnr_429.Rmd")

learnr_429.Rmd

---
title: "Hello, Tutorial!"
author: "J.J. Allaire"
date: "March 1st, 2017"
output: learnr::tutorial
runtime: shiny_prerendered
# Do not index/display tutorial by setting `private: true`
# private: true
description: >
  This is a demo tutorial.
---

```{r setup, include=FALSE}
library(learnr)
library(gradethis)
gradethis::gradethis_setup()
```

The following code computes the answer to 1+1. Change it so it computes 2 + 2:

```{r addition, exercise=TRUE}
1 + 1
```




```{r test, exercise=TRUE}
2+2
```

```{r test-solution}
#solution
2 + 2
```

```{r test-hint-1}
#hint1
```


```{r test-hint-2}
#hint2
```

```{r test-check}
#solution
gradethis::grade_code()
```


### unknown (`awk`)
```{awk awk-chunk, exercise=TRUE}

```


### custom
```{R addition-custom, exercise=TRUE, exercise.cap = "My Caption Here"}

```
### custom #2
```{R addition-custom-2, exercise=TRUE, exercise.cap = "<div style='color:green;'>My Green Caption</div>"}

```

### bash
```{bash bash-chunk, exercise = TRUE}

```

### c
```{c c-chunk, exercise = TRUE}

```

### cc
```{cc cc-chunk, exercise = TRUE}

```

### coffee
```{coffee coffee-chunk, exercise = TRUE}

```

### css
```{css css-chunk, exercise = TRUE}

```

### go
```{go go-chunk, exercise = TRUE}

```

### groovy
```{groovy groovy-chunk, exercise = TRUE}

```

### haskell
```{haskell haskell-chunk, exercise = TRUE}

```

### js
```{js js-chunk, exercise = TRUE}

```

### mysql
```{mysql mysql-chunk, exercise = TRUE}

```

### node
```{node node-chunk, exercise = TRUE}

```

### octave
```{octave octave-chunk, exercise = TRUE}

```

### perl
```{perl perl-chunk, exercise = TRUE}

```

### psql
```{psql psql-chunk, exercise = TRUE}

```

### python
```{python python-chunk, exercise = TRUE}

```

### r
```{r r-chunk, exercise = TRUE}

```

### rcpp
```{rcpp rcpp-chunk, exercise = TRUE}

```

### rscript
```{rscript rscript-chunk, exercise = TRUE}

```

### ruby
```{ruby ruby-chunk, exercise = TRUE}

```

### sass
```{sass sass-chunk, exercise = TRUE}

```

### scala
```{scala scala-chunk, exercise = TRUE}

```

### scss
```{scss scss-chunk, exercise = TRUE}

```

### sql
```{sql sql-chunk, exercise = TRUE}

```

Should look like:

[cpsievert]

Some other icons that might be worth adding: psql (postgresql), node, scala, ruby, sass, perl

[schloerke]

@cpsievert Added a line for each knitr engine, even if there is no icon. This way, they are at least accounted for.

[cpsievert]

🌮 🎉 🌮