Based on https://github.com/swcarpentry/lesson-example/pull/144/files

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.