Add more information to Link nodes (for round-trip rendering)

For #12, we need to preserve link reference definitions. We also need to be able to distinguish between the different link types (inline links, autolinks, reference links).

There is an old PR which might serve as a starting point: https://github.com/atlassian/commonmark-java/pull/20#issuecomment-142782665

I'm not sure yet what the final Node types should look like. Ideally we'd have an interface that all the links can implement for code that doesn't care about what the source representation was.

If someone is interested in working on this, please comment here first so we can figure out what the end result should look like.

Commencez par lire issue #12 et l’ancienne discussion du PR dans pull request #20, en particulier le commentaire lié dans l’issue. Clarifiez le modèle Node final et la manière dont les inline links, les autolinks et les reference links doivent être distingués. C’est terminé lorsque les définitions de références de liens et les types de liens source sont préservés pour le rendu round-trip.