Only cleaning intermediate R files on demand.

1.  Use site.root for root path if set, otherwise use site.github.url.
2.  Display extra files for workshop-template repo as well as for lessons.

1.  Moving all YAML library usage into 'util.py' via utility function.
2.  Wrapping 'import yaml' in error handling to report what library to install.

1.  Converting error reporting.
2.  Simplifying checking logic.
3.  Refactoring metadata loading.
4.  Modifying lesson checker to reflect refactoring.

Closes #47.

Closes #55.

1.  Splitting commands to clean intermediate R files into separate target.  These files must be committed to the repo, so they shouldn't be deleted under normal circumstances.
2.  Modifying intermediate file generation to create fig/rmd-* rather than fig/swc-rmd-*, since this template is used by other carpentries (not just SWC).

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.

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.