Get timelimit from complete exercise object

[gadenbuie]

Edit: on second thought, we were just setting timelimit too early. Getting the timelimit after filling out the exercise object fixes the issue in #492.

This fixes #492. The exercise reactive returned by input$`tutorial-exercise-%s-code-editor` doesn't include all of the exercise options. It seems that exercise_rx() only contains the label, the submitted code, a timestamp and flags for restore and should_check. The result is that, when setup_exercise_handler tries to find $options$exercise.timelimit, that field is always NULL because $options isn't a item in the reactive exercise list.

I'm not sure if that should be rectified in the exercise object logic, but I was able to fix the issue by merging the reactive exercise object with the cached exercise object that contains the complete options, check code, etc. (Edit: which we do later so we were just setting timelimit too eary.)

Here's a reprex Rmd for testing. Prior to the change in this PR, learnr ignores both the chunk and global options and always caps exercises at 30 seconds. Both time-limit setting methods are demonstrated in the reprex.

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

```{js, echo=FALSE}
// custom message handler to report elapsed time
Shiny.addCustomMessageHandler('time', function(x) {
  document.getElementById('section-time').textContent = x
})
```

## Time Limit

```{r setup}
library(learnr)
tutorial_options(exercise.timelimit = 15)
```

The code has been running for <span id="time">0</span> seconds.

```{r ex-setup}
sleep <- function(n) {
  session <- shiny::getDefaultReactiveDomain()
  session$sendCustomMessage("time", 0)
  for (i in 1:n) {
    Sys.sleep(1)
    session$sendCustomMessage("time", i)
  }
}
```

```{r ex, exercise = TRUE, exercise.timelimit = 5}
sleep(15)
```

PR task list:

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