1.  Create file `_includes/links.md` with definitions of all links.
2.  Replace links at the bottom of Jekyll'd `.md` files with inclusion of that file.
3.  Modify `bin/lesson_check.py` to load and check definitions in this file.

This eliminates most redundancy in link definitions, though
`CONTRIBUTING.md` and `README.md` still contain their own links, since
they are not Jekyll'd.  Note that we *cannot* put the links into
`_layouts/*.html` (or even `_layouts/something.md`) because of the
order in which Jekyll expands things - we have to explicitly include
it in each page.

1. Checking that `index.md` has `root` set to '.'
2. Checking that `_config.yml` has `root` set to '..'

An internally-defined Markdown link has the form [xxx][yyy].  If the
ID 'yyy' doesn't resolve, the text is left as-is, so we check for
that, then subtract those that reference configuration values using
'{{'.  It's a hack, but it'll catch at least a few things.

1.  Created `bin/repo_check.py`.
2.  Moved `require()` to `util.py`.
3.  Updated `Makefile` with new target.

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

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.