`generate_md_episodes()` is modified such that datasets storred into
_episodes_rmd/data/ needed to knit the episodes, are copied into
_episodes/data/, and are made available at the root of the site.

1.  Modify `read_markdown` to return list of (index, line, line_length).
2.  Modify `lesson_check.py` to make line-length checking optional.

1.  Renaming Ruby file to use underscore in name (consistent with other `bin` scripts).
2.  Generating `_includes/all_figures.html` for the first time when creating a new lesson.