use anonymous function to generate markdown files

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

Fix Rmd formatting

add episode example written in Rmarkdown

add tools to deal with episodes written in Rmd

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.