Add new anybox question type to solve #382

[elmstedt]

Solves #382

This creates a question type which has multiple correct questions but for which it is not required to select all of the correct options before proceeding. A successfully answered question will add messages indicating which correct responses were missed as well as which incorrect responses were selected.

In order to pass the additional arguments min_right and max_wrong to question_anybox(), I added them as attributes to the question object returned by learnr::question(). This feels a bit kludgy, but I wanted to not alter the structure of the question object to avoid any potential conflicts. Giving the object two additional attributes seemed the least invasive.

---
title: "Tutorial"
output: learnr::tutorial
runtime: shiny_prerendered
---

```{r setup, include=FALSE}
library(learnr)
knitr::opts_chunk$set(echo = FALSE)
```

## Topic 1

```{r two_right_one_wrong, echo = FALSE}
question_anybox(
  "Select at least two toppings that belong on a Margherita Pizza (and no more than one that doesn't):",
  answer("tomato", correct = TRUE, message = "Tomatoes too!"),
  answer("mozzarella", correct = TRUE, message = "Don't forget the cheese!"),
  answer("basil", correct = TRUE, message = "Basil gives it a distinctive flavor!"),
  answer("extra virgin olive oil", correct = TRUE, message = "You need olive oil too!"),
  answer("pepperoni", message = "Pepperoni is a great topping! ... just not on a Margherita Pizza"),
  answer("onions", message = "Onions!? No and yuck!"),
  answer("bacon", message = "Bacon doesn't belong here!"),
  answer("spinach", message = "Spinach? With Olive Oil? Only if you're Popeye!"),
  random_answer_order = TRUE,
  allow_retry = TRUE,
  try_again = "Be sure to select all four toppings!",
  min_right = 2,
  max_wrong = 1
)
```

PR task list:

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

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[schloerke]

I also don't know if this logic should be merged into the default checkbox logic, or if it should be made into a separate exported function.