Optionally reveal (or hide) exercise solution

[gadenbuie]

Adds exercise.reveal_solution, which can be set in the *-solution chunk options or globally with tutorial_options() or tutorial.exercise.reveal_solution. If FALSE, the solution is not revealed to the user. The local chunk option takes precedence over the global option.

Fixes #402

Demo Tutorial

---
title: "Optionally Reveal Exercise Solution"
output: learnr::tutorial
runtime: shiny_prerendered
---

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

## Set global reveal solution option
# options(tutorial.exercise.reveal_solution = FALSE)
# tutorial_options(exercise.reveal_solution = FALSE)
```

## Intro

### Default

<!-- exercise.reveal_solution needs to be on solution chunk -->

```{r default, exercise = TRUE, exercise.reveal_solution = TRUE}
1 + 1
```

```{r default-hint-1}
# hint code 1
```

```{r default-hint-2}
# hint code 2
```

```{r default-solution}
2
```

### Always Hides Solution

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

```{r hide-hint-1}
# hint code 1
```

```{r hide-hint-2}
# hint code 2
```

```{r hide-solution, exercise.reveal_solution = FALSE}
2
```

### Always Shows Solution

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

```{r show-hint-1}
# hint code 1
```

```{r show-hint-2}
# hint code 2
```

```{r show-solution, exercise.reveal_solution = TRUE}
2
```

PR task list:

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

[gadenbuie]

1. I called the option exercise.reveal_solution but I'm open to other names if there are any suggestions.
2. Unlike the other exercise options that are set on the exercise chunk, this option needs to be set on the *-solution chunk. I'm okay with that, since it's only relevant to the solution, but I thought I'd bring it up.
3. I'm not sure how to test this feature. learnr suggests shinytest but doesn't seem to use it in the test suite. Would this be a good use case for shinytest?

[schloerke]

LGTM given minor changes

[schloerke]

1. I like exercise.reveal_solution
2. See Optionally reveal (or hide) exercise solution #470 (comment)
3. (addressed)