Clarifying challenge format

Explaining why we use Markdown

Update in the list of contributors from the Git history.

Update in the list of contributors from the Git history.

AUTHORS: List contributors to this project

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.