Implement counter of errors at tools/check.py output

Sub-heading raise warning not error

Add getting ready

Replace #235.

This change was suggested by @abought.

Close #230.

Related with https://github.com/swcarpentry/lesson-template/issues/220.

Replace div with section and aside

to make the HTML more semantic.

No visual change was made.

Close #231.

Fix check of glossary item

CommonMark doesn't support definition list
and we have to workaround this limitation.

This commit removes the request
that line continuation start with four white spaces
because (1) CommonMark remove the white spaces and
(2) Pandoc doesn't care about it.

    $ pandoc <<EOF
    foo
    :   bar
        continue
    EOF
    <dl>
    <dt>foo</dt>
    <dd>bar continue
    </dd>
    </dl>
    $ pandoc <<EOF
    foo
    :   bar
    continue
    EOF
    <dl>
    <dt>foo</dt>
    <dd>bar continue
    </dd>
    </dl>

consolidate badge classes to swc.css

Thanks to ocefpaf for request that.

Add variable to represent Pandoc

Pandoc had some changes in the last versions.
Having a variable for Pandoc will help
maintainers to handle different versions of Pandoc.

Figures should be centered
and not overflow the screen.

Add article tag as wrap for lesson content

CONTRIBUTING.md: Remove the maintainer references

README.md: Add local maintainer listing

This is the new plan [1], instead of referring to a central listing
from CONTRIBUTING.md.

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-89643048

Because we're going to be storing maintainer information locally in
the README.md [1].

[1]: https://github.com/swcarpentry/lesson-template/issues/195#issuecomment-89643048

Remove logger name (root) from console messages