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>