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.

Lies zunächst issue #12 und die alte PR-Diskussion in pull request #20, insbesondere den im Issue verlinkten Kommentar. Kläre das endgültige Node-Modell und wie inline links, autolinks und reference links unterschieden werden sollen. Als abgeschlossen gilt die Aufgabe, wenn Link-Referenzdefinitionen und Quell-Linktypen für das Round-Trip-Rendering erhalten bleiben.