Consider creating a function to access configuration values

[ocaisa]

Right now we use things like

`r config$remote$prompt` module list

but this just returns NULL when the configuration variable does not exist (i.e., it does not error).

I wonder if we are not better off creating a helper function where we do some error control:

get_config <- function(..., default = '', required = TRUE) {
  path <- list(...)
  value <- config

  for (name in path) {
    if (!is.list(value) || is.null(value[[name]])) {
      if (!required) return(default)
      stop(
        sprintf("Missing configuration value: %s", paste(path, collapse = "$")),
        call. = FALSE
      )
    }
    value <- value[[name]]
  }
  value
}

which could have usage like

`r get_config("remote", "prompt")` module list

This would also open the door to incremental improvement.