Add gradethis_default() lookup helper

[gadenbuie]

Currently, we lookup gradethis options via getOption("gradethis.fail", "Fall back default"), which has a few disadvantages:

• the correct fallback needs to be used in every case
• it clutters the API since the preferred method for setting the defaults is via gradethis_setup()
• it isn't easy to differentiate between actively supplied arguments and default values determined from global settings without resorting to sentinel values that further obscure the mechanics

We should add a helper for looking up global default values: gradethis_default()

fail <- function(
  message = gradethis_default("fail"),
  ...,
  hint = gradethis_default("fail.hint"),
  encourage = gradethis_default("fail.encourage")
) { ... }

where the documentation can better direct users to gradethis_setup() where those values are set since both functions would be documented in the same help page.

Internally, gradethis_default() could add an attr() that we could use to determine whether or not the argument value was supplied by the user or looked up from global options.

[gadenbuie]

I implemented a version of this that — rather than taking the option as a character argument as in gradethis_default("<option>") — is instead a list of functions that access the current default value. The list is named gradethis_settings and its structure enables auto-complete:

I was thinking about a developer-facing interface at the time, hence gradethis_settings rather than gradethis_default to avoid clashes with our internal list of default values.

With a little extra work, we could allow the functions to also operate as setters when provided with a value.

# Set fail.hint default value
gradethis_settings$fail.hint(TRUE)

If we export this object, then the function signature for fail() (and others) would look like this:

fail <- function(
  message = gradethis_settings$fail(),
  ...,
  hint = gradethis_settings$fail.hint(),
  encourage = gradethis_settings$fail.encourage()
) { 
  # ... 
}

I'm not completely sold on the name, gradethis_config or gradethis_options also come to mind, but I went with gradethis_settings since it's closer to gradethis_setup().