[fix #270] check knitr version before converting Rmd to md

Removed lingering references to motivation.md and motivation.html

Removing Legend from required headings

Removing support for motivational slideshows

on a suggestion of @sritchie73 and @aammd

Avoid wrong error with links at index.md

Modifying page template so that main title links to home page.

If there is a subtitle, the main title links to the `index.html` home page.  If there is no subtitle, this *is* the home page, so there is no link.

We don't validate links to glossary
but we validate links from index.md
to ensure that their text
and the title of the lesson it point to
are the same.

This exclude links point to one address with '#'
since it probably points to a glossary entry.

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.

Figures should be centered
and not overflow the screen.

Add article tag as wrap for lesson content

CONTRIBUTING.md: Remove the maintainer references