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.

1.  Moving YAML parsing to bin/util.py.
2.  Adding bin/extract_figures.py.
3.  Adding Makefile targets to regenerate image inclusion file.
4.  Adding _extras/figures.md to display all images.