Only try building R files if there are some

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.

Closes #151.

Closes #250.

Fix Rmd formatting

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.

update with new github import

Include instructions for setting "HOME" on Windows