Close #178

See https://github.com/swcarpentry/workshop-template/issues/410

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.

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.