feat: adding option_fn field to option_spec

[dgkf]

Adds option_fn, a function that may be applied to the return value of an option before its value is retrieved.

Capabilities made possible:

• Configuring an option as a function or quote, but allowing opt() to return the result of calling the function without having to consistently apply the function or wrap it in a specialized helper.
• Adding side effects to access via opt()
• Coercing values into a consistent type before being retrieved
• Specializing error messages when an option does not have an expected format

Behaviors still under consideration:

• Should this function apply to the default value? For consistency I'm leaning toward yes.
• The expected signature should certainly accept the value, but beyond that I'm not totally sure. For now (considered experimental in the docs) I pass all relevant info related to retrieving the value, but this feels a bit excessive.

---

@tdeenes, your feedback would be greatly appreciated if you can take a look

[tdeenes]

I like the idea that we can pass arbitrary arguments to option_fn via .... However, if we allow for this, two other changes are required:

1. The doc #' @param ... Additional arguments unused must be updated in line 8;
2. A more thorough check of option_fn is required, otherwise the user can get weird error messages or even worse, inconsistent failures/wrong results;
3. Depending on how strict we want to be with the signature of option_fn, we might introduce a little overhead here and check what arguments to pass to option_fn:

.call <- as.call(
  c(
    list(spec$option_fn$fn, value),
    alist(x = x, default = default, env = env, ... = ..., source = source)[spec$option_fn$args]
  )
)
eval(.call)

See my comment at option_spec for the proposed utility function.

[tdeenes]

Nice! There are a few typos in the doc which shall be fixed. I also added two optional comments on the use of ... in option_fn.

[dgkf]

Fixing spelling - thanks @tdeenes

[tdeenes]

@dgkf I think you forgot to update the roxygen doc:

#' @param ... Additional arguments unused

This should be modified to:

#' @param ... Arguments passed to `option_fn`