Pull Request: carpentries/lesson-example#225

* replace sub-heading with sub-title

that is missing formating.

Close swcarpentry/lesson-example#195

The 'input' (raw) block showed the `.source` class identifier, but the rendered output used `.language-python`.

Close #134

Close #81

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.  Reporting line numbers for errors.
2.  Checking classes of code blocks.
3.  Tidying up omissions found by running against example lesson.