Fix loading placeholder when question text is HTML

[gadenbuie]

question() accepts HTML for the text argument, in particular using htmltools tags. But the default value of the loading argument uses c() to concatenate c("Loading...", text), which results in an unusual list that quiz_text() isn't expecting and markdown::renderMarkdown() throws an error.

question(
  htmltools::tags$p("Did this work?"),
  answer("yes"),
  answer("no", correct = TRUE)
)

## Error in renderMarkdown(file, output = NULL, text, renderer = "HTML",  : 
##   argument "file" is missing, with no default

I just changed the default to c("Loading", format(text)) so that the shiny tags are formatted as a character.

PR task list:

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

I'll add a NEWS item if the PR looks good to you.

[schloerke]

LGTM!

[gadenbuie]

I found another related case where a non-character answer() text causes issues

question(
  text = "1+1",
  answer(2, correct =TRUE)
)
## Error in renderMarkdown(file, output = NULL, text, renderer = "HTML",  : 
##   argument "file" is missing, with no default

and I added https://github.com/gadenbuie/learnr/blob/d94855cfd92327273909472e9c5e2f457d5fe73e/R/quiz.R#L227-L229 in quiz_text() before the text is handed off to markdown::markdownToHTML().