simplify to single _episodes_rmd/data folder

    if (!require("stringr"))

        stop("The package stringr is required for generating the lessons.")

    ## where the Rmd files and the datasets are located

    rmd_path <- "_episodes_rmd"

    rmd_data <- file.path(rmd_path, "data")

    ## where the markdown files and the datasets will end up

    dest_path <- "_episodes"

    dest_data <- file.path(dest_path, "data")

    ## find all the Rmd files, and generates the paths for their respective outputs

    src_rmd <- list.files(pattern = "??-*.Rmd$", path = rmd_path, full.names = TRUE)

    dest_md <- file.path(dest_path, gsub("Rmd$", "md", basename(src_rmd)))

    src_rmd <- list.files(pattern = "??-*.Rmd$", path = "_episodes_rmd", full.names = TRUE)

    dest_md <- file.path("_episodes", gsub("Rmd$", "md", basename(src_rmd)))

    ## knit the Rmd into markdown

    mapply(function(x, y) {

        knitr::knit(x, output = y)

    }, src_rmd, dest_md)

    ## copy the datasets from _episodes_rmd/data to _episodes/data

    rmd_data_files <- list.files(path = rmd_data, full.names = TRUE)

    dest_data_files <- file.path(dest_data, basename(rmd_data_files))

    if (!dir.exists(file.path(dest_data)))

        dir.create(file.path(dest_data))

    apply(cbind(rmd_data_files, dest_data_files), 1,

          function(x) file.copy(x[1], x[2]))

}

generate_md_episodes()