Get the full resource list of markdown files

Usage

get_resource_list(path, trim = FALSE, subfolder = NULL, warn = FALSE)

path: path to the lesson
trim: if TRUE, trim the paths to be relative to the lesson directory. Defaults to FALSE, which will return the absolute paths
subfolder: the subfolder to check. If this is NULL, all folders will checked and returned (default), otherwise, this should be a string specifying the folder name in the lesson (e.g. "episodes").
warn: if TRUE and subfolder = "episodes", a message is issued to the user if the episodes field of the configuration file is empty.

a list of files by subfolder in the order they should appear in the menu.