Signed-off-by: Maxim Belkin <maxim.belkin@gmail.com>

If _extras/figures.md exists,

    $ ./bin/lesson_initialize.py

will return

    Warning: _extras/figures.md already exists.
    **Exiting without creating files.**

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.  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.