Find stem expressions in dlist items

Fixes #8 (closed)

dlist items are not blocks themselves, but a list of [terms, description] arrays.

Therefore we need to visit all terms and the description (if it exists) in each Dlist item.

Reference:

• https://github.com/asciidoctor/asciidoctor/blob/v2.0.17/lib/asciidoctor/list.rb#L45
• https://github.com/asciidoctor/asciidoctor/blob/v2.0.17/lib/asciidoctor/converter/html5.rb#L558